Rat hepatocyte cultures (RL-PR-C) were tested with the (+)- or (-)-trans-7,8-dihydrodiol isomers of benzo(a)pyrene [B(a)P] and were assessed for growth inhibition, chromosomal damage, growth in soft agar, and tumor formation. Because early-passage cells have a noninducible low specific activity, aryl hydrocarbon hydroxylase parallel inhibition studies were performed on aryl hydrocarbon hydroxylase-inducible late-passage cultures. Early-passage cells exhibited little inhibition in the presence of B(a)P or either isomer. Comparable later-passage cells demonstrated inhibitory effects with B(a)P and the (+)- and (-)-trans-7,8-dihydrodiol metabolites. No treated cultures grew in soft agar, and the modal number of chromosomes was unaffected by carcinogen treatment. However, both (+) and (-) isomer-treated early-passage cells formed tumors in isogeneic animals, the (+) isomer being more efficient in this regard. These results indicate that noninhibitory doses of either the (+)- or (-)-trans-7,8-dihydrodiol isomer of B(a)P are nonetheless capable of malignantly transforming hepatocyte cells in vitro.