1,2-Dibromo-2,4-dicyanobutane is used in cosmetics and other household products. 1,2-Dibromo-2,4-dicyanobutane was nominated for study by the National Institute of Environmental Health Sciences because of its widespread use as a component of numerous over-the-counter health care products. Male and female F344/N rats and B6C3F1 mice received 1,2-dibromo-2,4-dicyanobutane (greater than 99% pure) in acetone (2-week and 3-month studies) or 95% ethanol (2-year studies) by dermal administration for 2 weeks, 3 months, or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium, Escherichia coli, and mouse peripheral blood erythrocytes. 2-WEEK STUDY IN RATS: Groups of five male and five female rats were dermally administered 0, 37.5, 75, 150, 300, or 600 mg 1,2-dibromo-2,4-dicyanobutane/kg body weight in acetone, 5 days per week for 16 days. All male and female rats survived to the end of the study. Mean body weights of dosed male and female rats were similar to those of the vehicle controls. Irritation, thickened skin, and ulcers were observed at the site of application in most dosed males and females. The thyroid gland weights of males administered 600 mg/kg were significantly less than those of the vehicle controls. The liver and kidney weights of 300 and 600 mg/kg females were significantly increased. A spectrum of nonneoplastic lesions including epidermal hyperplasia and hyperkeratosis, sebaceous gland hyperplasia, and dermal chronic active inflammation occurred at the site of application in all dosed groups of rats. Necrosis, ulcer, and parakeratosis of the epidermis occurred in most dosed groups of rats. 2-WEEK STUDY IN MICE: Groups of five male and five female mice were dermally administered 0, 75, 150, 300, 600, or 1,200 mg/kg 1,2-dibromo-2,4-dicyanobutane in acetone, 5 days per week for 16 days. All male and female mice survived to the end of the study. The final mean body weight of 300 mg/kg males was significantly less than that of the vehicle controls. Hyperactivity was observed in all dosed groups of mice. Irritation, thickened skin, and ulcers were observed at the site of application in dosed mice. The liver weights of 600 and 1,200 mg/kg males and 1,200 mg/kg females were significantly increased relative to those of the vehicle control groups. The heart weights of 600 and 1,200 mg/kg males and the kidney weights of 150 and 600 mg/kg males were significantly increased. The thymus weights of males administered 300 mg/kg or greater and those of all dosed groups of females were significantly decreased. Skin lesions at the site of application including epidermal hyperplasia, hyperkeratosis, parakeratosis, necrosis, and ulcers; dermal chronic active inflammation; and sebaceous gland hyperplasia occurred in all dosed groups of mice. Necrosis of the dermis occurred in most dosed groups of mice. 3-MONTH STUDY IN RATS: Groups of 10 male and 10 female rats were dermally administered 0, 0.2, 0.6, 2, 6, or 18 mg/kg 1,2-dibromo-2,4-dicyanobutane in acetone, 5 days per week for 14 weeks. All male rats survived to the end of the study. One 2 mg/kg female rat died on day 91. Mean body weights of dosed male and female rats were similar to those of the vehicle controls. Clinical findings of toxicity included thin hair coat in male and female rats and irritation at the site of application in males. At the site of application, the incidences of epidermal hyperplasia in males administered 0.6 mg/kg or greater and females administered 2 mg/kg or greater and the incidences of epidermal hyperkeratosis in all dosed groups of rats were significantly increased. In the dermis at the site of application, the incidences of chronic active inflammation in 6 and 18 mg/kg males and females administered 2 mg/kg or greater and sebaceous gland hyperplasia in males administered 6 or 18 mg/kg were significantly increased. 3-MONTH STUDY IN MICE: Groups of 10 male and 10 female mice were dermally administered 0, 0.2, 0.6, 2, 6, or 18 mg/kg 1,2-dibromo-2,4-dicyanobutane in acetone, 5 days per week for 14 weeks. All mice survived to the end of the study. Mean body weights of dosed male and female mice were similar to those of the vehicle controls. Irritation at the site of application was increased in male mice administered 18 mg/kg. The liver and lung weights of dosed females were generally significantly less than those of the vehicle control group. The incidences of minimal to mild epidermal hyperplasia and hyperkeratosis at the site of application were significantly increased in male and female mice administered 2 mg/kg or greater. The incidence of epidermal necrosis in 18 mg/kg males and the incidences of epidermal parakeratosis in 6 and 18 mg/kg males were significantly increased. In the dermis, the incidences of minimal to mild chronic active inflammation in 18 mg/kg males and in females administered 2 mg/kg or greater and fibrosis in 18 mg/kg males and females were significantly increased. The incidences of sebaceous gland hyperplasia at the site of application were significantly increased in males and females administered 6 or 18 mg/kg. 2-YEAR STUDY IN RATS: Groups of 50 male and 50 female rats were dermally administered 0, 2, 6, or 18 mg/kg 1,2-dibromo-2,4-dicyanobutane in 95% ethanol, 5 days a week for 104 to 105 weeks. Survival of males administered 18 mg/kg was significantly greater than that of the vehicle controls. Body weights of 18 mg/kg males and females were 7% less than those of the vehicle control groups after 1 year. Irritation at the site of application was reported in most males and females administered 6 or 18 mg/kg. There were no increases in the incidences of neoplasms in dosed rats. At the site of application, the incidences of epidermal hyperplasia in males and females administered 6 or 18 mg/kg and the incidences of hyperkeratosis of the epidermis in all dosed groups were significantly increased. The incidences of minimal to mild inflammation in the dermis at the site of application were significantly increased in males administered 6 or 18 mg/kg and in all dosed groups of females. The incidence of epidermal necrosis at the site of application in 18 mg/kg females was significantly increased. The incidences of inflammation of the nose were significantly increased in all dosed groups of male rats. The combined incidence of mammary gland fibroadenoma, adenoma, or adenocarcinoma occurred with a negative trend, and the incidence was significantly decreased in 6 mg/kg female rats. 2-YEAR STUDY IN MICE: Groups of 50 male and 50 female mice were dermally administered 0, 0.6, 2, or 6 mg/kg 1,2-dibromo-2,4-dicyanobutane in 95% ethanol, 5 days a week for 105 weeks. Survival of male and female mice was similar to that of the vehicle controls. Body weights of male and female dosed groups were similar to those of the vehicle control groups. No clinical findings were attributed to administration of 1,2-dibromo-2,4-dicyanobutane. There were no increases in the incidences of neoplasms in dosed mice. At the site of application, the incidences of minimal to mild hyperplasia of the epidermis were significantly increased in 2 and 6 mg/kg males and in all dosed groups of females. The incidences of minimal to mild chronic active inflammation in the dermis were significantly increased in all dosed groups of females. RESULTS 1,2-Dibromo-2,4-dicyanobutane was not mutagenic in any of several strains of Salmonella typhimurium or Escherichia coli when tested with and without hamster and/or rat liver metabolic activation enzymes (S9). In addition, no increase in the frequency of micronucleated erythrocytes was observed in male or female mice treated for 3 months with 1,2-dibromo-2,4-dicyanobutane by dermal application in acetone, indicating no potential for inducing chromosomal alterations in dividing cells in this test system. CONCLUSIONS Under the conditions of these 2-year dermal studies there was no evidence of carcinogenic activity of 1,2-dibromo-2,4-dicyanobutane in male or female F344/N rats administered 2, 6, or 18 mg/kg. There was no evidence of carcinogenic activity of 1,2-dibromo-2,4-dicyanobutane in male or female B6C3F1 mice administered 0.6, 2, or 6 mg/kg. 1,2-dibromo-2,4-dicyanobutane administration induced nonneoplastic lesions at the site of application in male and female rats and mice.