Weanling male mice with or without prior treatment with CCl4 (0.5 ml/kg) were exposed to either single (20.5, 25.6, 32 and 40 mg/kg) or multiple (4 mg/kg for 10 days or 8 mg/kg for 5 days) doses of aflatoxin B1. Evidence of its acute toxic and carcinogenic effects was found only in CCl4 pre-treated mice. Single dose LD 50 value under these conditions was calculated to be 26.8 mg/kg. Histological changes which predominantly occurred in the liver were characterized by acute necrosis and degeneration of the parenchymal cells. Hepatomas were observed as early as 9 months of age in CCl4 pre-treated animals given 4 mg/kg aflatoxin daily for 10 days. It is thus suggested that the repeated use of CCl4 as an anthelmintic in animals which are apparently resistant to the lethal and hepatocarcinogenic action of aflatoxin might increase their sensitivity to this mycotoxin.