A carcinogenesis bioassay of caprolactam, a chemical intermediate used in the production of nylon 6, was conducted by feeding diets containing 3,750 or 7,500 ppm caprolactam to groups of 50 male or female F344 rats and 7,500 or 15,000 ppm to groups of 50 male or female B6C3F1 mice for 103 weeks. Control groups consisted of 50 undosed rats and 50 undosed mice of each sex. Throughout the bioassay, mean body weight gains for dosed rats and mice of either sex were decreased when compared with those of the controls. No other compound-related effects were observed. Under the conditions of this bioassay, caprolactam was not carcinogenic for F344 rats or B6C3F1 mice. Levels of Evidence of Carcinogenicity: Male Rats: Negative Female Rats: Negative Male Mice: Negative Female Mice: Negative Synonyms: aminocaproic lactam; 2-oxohexamethylenimine