The cytotoxic and mutagenic effect of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti BPDE) in normally excising diploid human cells treated just prior to onset of S was compared with that of cells allowed approximately 16 h for excision repair before onset of S and with that observed in excision-deficient xeroderma pigmentosum (XP12BE) cells. The cells were synchronized by release from density inhibition of cell replication. DNA synthesis began approximately 22 h after the cells were plated at lower density (i.e., 1.4 x 10(4) cells/cm2). The frequency of thioguanine-resistant mutants induced in normal cells treated just prior to onset of S was approximately 12- to 16-fold higher than that observed in cells treated in early G1 or treated in G0 (confluence) and then plated at lower density. The frequency approximated that expected for XP12BE cells from extrapolation of data obtained at lower doses. The frequency of mutants measured in normal cells treated in exponential growth was also much higher than that in the cells treated in early G1 or in G0. No such difference could be seen in XP12BE cells treated in exponential growth or in G0. In contrast to the mutagenicity data in the normal cells, there was no significant difference in the slope of the survival curve of normal cells treated at various times prior to S phase at low densities. However, normal cells treated even at the onset of S exhibited survival equal to XP12BE cells given a 4- to 5-fold lower dose. The data support the hypothesis that DNA synthesis is the cellular event which converts unexcised DNA lesions into mutations. However, they indicate that S is not the event primarily responsible for translating DNA damage into cell death. Accompanying studies on the rate of excision of anti BPDE adducts from the normal cells during the period prior to S support the conclusions.