The immunotoxic effects of the environmental carcinogen benzo[a]pyrene (BaP) have been evaluated using mouse, but not human, lymphocytes. Since susceptibility to immunomodulation by BaP may be species-related, the effects of BaP on human peripheral blood mononuclear cells (HPBMC) from several individuals were investigated in vitro. HPBMC were stimulated by the T-cell mitogen concanavalin A (con A) in the presence of BaP (10(-9) to 10(-6) M) or vehicle control (acetone) and evaluated after 3 days for [3H]TdR incorporation, cell numbers, cell-cycle distribution, viability, and expression of cell surface or intracellular activation antigens. IL-2 production was quantified at 24 hr. Incorporation of [3H]TdR was the parameter most sensitive to BaP with an IC50 relative to controls of 4.5 +/- 3.1 x 10(-8) M with a range for individuals of 1.8 x 10(-8) to 6.6 x 10(-8) M (n = 15). The P450 inhibitor 7,8-benzoflavone (10(-6) M) partially prevented BaP inhibition of [3H]TdR incorporation. Maximum suppression of this parameter required that BaP be present in culture > 24 hr. The number of cells recovered from culture was decreased by 10(-8) to 10(-6) M BaP, but viability was only slightly diminished by 10(-7) to 10(-6) M BaP. BaP had little or no effect on the percentages of cells with induced IL-2 (CD25) or transferrin (CD71) receptors or on the amount of IL-2 activity present in 24-hr culture supernatants. Despite the BaP-mediated decrease in [3H]TdR incorporation, cell-cycle analysis indicated that BaP at 10(-7) and 10(-6) M increased the percentages of cells in S phase, and correspondingly decreased the percentages of cells in the G0/G1 phase. Also, BaP (10(-7) to 10(-6) M) decreased the percentages of G0/G1 cells that were positive for the intracellular activation antigen PCNA (or Ki-67), but had no affect on this percentage if cells were prevented from traversing S phase by mimosine or aphidicolin. These results suggest that BaP inhibits DNA synthesis and proliferation of con A-stimulated, human peripheral blood T-cells at culture concentrations as low as 10(-8) M by a P450-dependent process, but has little or no effect on measured G1-associated events at culture concentrations up to 10(-6) M.