The transplacental and direct action of benzo(a)pyrene (BP) on mice of strains A and C57BL and of their progeny was studied. BP was found to represent a carcinogenic risk for the progeny. The greatest carcinogenic effect in progeny of strain A mice was exhibited by BP in a dose of 6 mg: The frequency of development of lung tumors was 76.8% compared with 12.3% in the control (P<0.001). Liver tumors were found in the progeny of the C57BL mice (chiefly in males). Their frequency after a dose of 12 mg of BP was 31.6% in males and 9.1% in females, compared with only 1.2% in males in the control. No tumors of the liver were observed in females in the control. Pyrene, the noncarcinogenic analog of BP, had no carcinogenic effect.