A polybrominated biphenyl fire retardant (Firemaster FF-1) was responsible for the widespread environmental contamination and animal losses in Michigan during 1973 and 1974. In Fischer 344/N rats orally given 30,100,300, and 1,000 mg/kg (5 days/week, 22 total doses) for 4.5 weeks and observed for 90 days after the start of treatment, the LD50 was determined to be 65 mg/kg/day (total 1.43 g/kg) for the female rat and 149 mg/kg/day (total 3.28 g/kg) for the male. All female rats given the dosage of 100 mg/kg/day (22 doses, total 2.20 g/kg) died between 41 and 53 days after the start of treatment, whereas 38% of the males died between 50 and 73 days. Pathologic changes in treated rats were large liver, accentuation of the hepatic lobular markings, and atrophy of thymus and spleen. Microscopically, hepatic changes were characterized by congestion, fatty metamorphosis, and multifocal liquefactive necrosis. Male rats given 100 mg/kg/day and dying after 90 days had subacute to chronic hepatitis with marked focal proliferation of bile ducts. Exposure to Firemaster FF-1 may produce atypical liver nodules in the rat as early as 6 months after they were first given the preparation. Marked hepatotoxic effect persisted in surviving rats when examined after 6 months.