Studies have demonstrated that macrophages are the cell types which metabolize benzo[a]pyrene (B[a]P) within the murine spleen. B and T cells, polymorphonuclear cells, or the splenic capsule did not produce amounts of B[a]P metabolites above those of background. Exposure of mice to B[a]P, a known inducer of isozymes of cytochrome P450, resulted in an increase in the amounts of some B[a]P metabolites generated by macrophages. Evaluation of the in vitro plaque-forming-cell response to several T-cell and macrophage-dependent antigens following in vivo exposure to B[a]P indicated that the macrophage is the cell type responsible for B[a]P-induced immunosuppression. While suggestive, the reported data have not definitively established that an enriched splenic macrophage population can generate 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydro-B[a]P (BPDE) from B[a]P-7,8,-dihydroxy-7,8-dihydrodiol (B[a]P-7,8-diol). These data are critical to our hypothesis that the splenic cell type(s) which form BPDE will be the primary target cell responsible for B[a]P-induced immunosuppression. The first objective was to determine if splenic macrophages could generate BPDE. Enriched (80-90% purity) populations of macrophages were incubated with [3H]B[a]P for 24 hr. BPDE generated was quantitated by analysis of the cis- and trans-tetrol hydrolysis products of BPDE via HPLC procedures. Splenic macrophages generated BPDE from B[a]P. The cis syn was the predominate tetrol detected. Exposure of mice to B[a]P increased the amounts of the trans-anti-tetrols 2.2-fold, the trans-syn-tetrols 2.0-fold, the cis-anti-tetrols 1.8-fold, and the cis-syn-tetrols 2.6-fold above those formed by macrophages of vehicle-exposed mice. Both cytochrome P450- and peroxyl radical-dependent pathways are known to oxidize B[a]P-7,8-diol to BPDE. Since macrophages were found to generate BPDE, the second objective was to investigate which enzymatic pathway was responsible for its formation. The B[a]P-(+)-7,8-diol isomer has been shown to produce different specific BPDE isomers via the cytochrome P450 and peroxyl radical pathways. Macrophage populations were incubated with [3H]B[a]P-(+)-7,8-diol for 24 hr and the contribution of the cytochrome P450 and peroxyl radical pathways to BPDE formation determined by detection of syn-BPDE-hydrolysis and anti-BPDE-hydrolysis products, respectively. Based on the ratio of anti/syn BPDE-derived tetrol products, the results demonstrate that splenic macrophages can generate the BPDE by both a cytochrome P450-dependent and -independent (peroxyl radical) pathway. Macrophages are the cells which metabolize B[a]P within the murine spleen and may be the cell type responsible for B[a]P-induced suppression of humoral immune responses.