A hamster tracheal cell line rapidly absorbed and subsequently metabolized benzo[a]pyrene (B[a]P). At 10(-7) M B[a]P, less than 5% of the applied hydrocarbon was retained by the cell after 8 h incubation. However, at higher concentrations up to 20% was retained. During a subsequent 24 h incubation in the absence of polycyclic hydrocarbon, the residual B[a]P was metabolized and, for the most part, excreted. Analysis of B[a]P bound to DNA showed that alkylation increased as a consequence of metabolism of the B[a]P retained after replacement of the culture medium. Analysis of DNA repair is therefore markedly affected by the contribution from this continued alkylation. At 10(-7) M B[a]P, DNA alkylation was rapidly completed, and within 24 h a marked reduction in total adducts was observed. During this phase, a specific removal was observed of two adducts that have been tentatively identified as derived from deoxyadenosine. A slower second phase of repair was followed for up to 5 days at which time 25% of the adducts still remained in DNA. During this slow phase, the repair appeared to show preference for two of the four remaining adducts. In cells that were incubated for 8 h with 10(-6) M B[a]P, no rapid early phase of repair was seen during the following 24 h because of the continued alkylation. Thereafter, only a slow repair was observed. The deoxyadenosine adducts were still detectable 5 days after treatment suggesting that their repair was inhibited or saturated at this high concentration of B[a]P.