Pulmonary macrophages (PAM) metabolically activated benzo[a]pyrene [B(a)P] and its proximate carcinogenic metabolite, (±)trans 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (7,8-diol), to ultimate mutagens that were detected in cocultivated Chinese hamster V79 cells. Increases in the frequency of ouabainresistant (Or) mutations and sister chromatid exchanges were found in V79 cells only when they were cocultivated with both PAM and the chemical procarcinogens. 7,8-Diol caused higher frequencies of both Or mutations and sister chromatid exchanges than did the parent compound, B(a)P. When metabolically activated by PAM the mean Or mutation frequency caused by B(a)P was 9 Or mutants/106 surviving V79 cells per 106 PAM and a 10-fold interindividual variation (range, 2-21) was found. The mean Or mutation frequency caused by 7,8-diol was 64 and a ninefold interindividual variation (range, 14-120) was found. In the absence of PAM, the Or mutation frequency in V79 cells was one or less Or mutant per 106 survivors. 7,8-Benzoflavone, an inhibitor of mixed function oxidases, reduced the frequencies of Or mutations and of sister chromatid exchanges in V79 cells caused by 7,8-diol and B(a)P. As expected 7,8-benzoflavone did not influence the frequency of Or mutations caused by one of the ultimate mutagens derived from B(a)P and 7,8-diol, (±)7β, 8α-dihydroxy-9α, 10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene. These data are consistant with the hypothesis that PAM may play a role in the activation of environmental chemical procarcinogens.