A study of the effect in rats of dichlorodiphenyl trichloroethane (DDT) on hepatocarcinogenesis that is initated by aflatoxin B1 (AFB1). In the first experiment, Buffalo rats were given a single oral dose of AFB1 (5 mg/kg) followed by dietary DDT (100 ppm) for 20 weeks. Neoplastic nodules were observed in 1 of the 14 AFB1-exposed rats, compared with 3 of the 19 rats in the AFB1/DDT group. In the second experiment, Wistar rats were given dietary aflatoxin B, (4 ppm) for 6 weeks followed by a 6-week exposure to DDT (500 ppm) in a plain semisynthetic diet. Five altered hepatic foci were displayed by seven rats in the AFB1 group, compared with 6 foci and one neoplastic focus in five of the AFB1/DDT rats at 32 weeks. Subsequently, the AFB1 group produced 8 (27.5%) tumor-bearing rats while 10 of the 28 (35.7%) AFB1/DDT-exposed rats were tumor-bearing by 60 weeks. The results suggest that DDT slightly potentiates hepatocarcinogenesis induced by either a single dose of AFB1 or short term-dietary AFB1.