The combined action of 7,12-dimethylbenz[a]anthracene (DMBA) and alpha-naphthoflavone (alpha NF) on the survival and neoplastic transformation of C3H10T1/2 mouse embryo fibroblasts has been examined and correlated with DNA adduct formation and removal. When a 24 h DMBA treatment of asynchronously growing cells was followed for the next 24 h by a treatment with alpha NF + DMBA, both killing and transformation per viable cell were abrogated to a large extent. In some instances, transformation was completely abrogated--i.e. reduced to control frequencies--even at nontoxic concentrations of DMBA, indicating that changes in survival were not the reason for the reduction in transformation. Even at toxic concentrations of DMBA, post-treatment with alpha-NF + DMBA resulted in 10-fold reductions in transformation frequency. 3-Methylcholanthrene (3MC) also reversed DMBA cytotoxicity but with a dependence on 3MC concentration that was qualitatively different from that for alpha NF. The abrogation of cell killing occurred at lower molar ratios of alpha NF:DMBA than the abrogation of transformation; less than or equimolar concentrations resulted in maximal abrogation of killing, but about equal concentrations were required to abrogate transformation. Although the preceding findings suggest that different mechanisms may be involved in these endpoints, taken together they suggest that second treatments make apparent the repair of lesions due to a first treatment with DMBA alone. To test this hypothesis, the formation and removal of DMBA-DNA adducts were measured. Adducts were not removed when the second treatment was growth medium alone, but enhanced removal was observed when second treatments consisted of DMBA alone or DMBA plus one of several other polycyclic aromatic hydrocarbons (PAHs). Relative to killing and neoplastic transformation, these results suggest DMBA induces a repair process that limits its own effectiveness--a process that can be sustained by other PAHs.