The immunosuppressive effects of polycyclic aromatic hydrocarbons (PAHs) on immune responses in rodents, both in vivo and in vitro, have been widely documented. However, few studies have addressed the immunotoxicity of PAHs in the human system. In this report, we examined the toxic effects of nine different PAHs on human peripheral blood T cell mitogenesis. We found that benzo(a)pyrene (BaP), 3-methylcholanthrene (3-MC), and 7,12-dimethylbenz(a)anthracene (DMBA) were highly immunotoxic in the human system, while dibenz(a,c)anthracene (DAC) and dibenz(a,h)anthracene (DAH) were of intermediate toxicity, 9,10-dimethylanthracene (DMA), benzo(e)pyrene (BeP), and benz(a)anthracene (BA) were mildly immunotoxic, and anthracene (ANTH) had no measureable toxicity at the concentrations tested. Our results using human lymphocytes differed from previous studies in rodents, in that BaP and 3-MC were the most immunotoxic PAHs in the human mitogenesis assay, while DMBA has long been regarded as the PAH that is most potently toxic to rodent T cell responses. We also showed that alpha-naphthoflavone (ANF), which functions as both an Ah receptor antagonist and an inhibitor of cytochrome P450 activity, was able to block the suppressive effects of both BaP and DMBA, but not 3-MC. This suggests that the immunotoxicity of 3-MC may be mediated through a different mechanism than BaP or DMBA. Addition of four different BaP metabolites directly to cultures of human mononuclear cells showed that the 7,8-dihydrodiol metabolite was the most toxic, and that this toxicity could be completely blocked by equimolar and 10-fold greater concentrations of ANF. The 7,8-dihydrodiol metabolite was probably further metabolized to the 7,8-diol epoxide, the toxicity of which could not be effectively reversed by ANF. The 4,5-epoxide metabolite was apparently cytotoxic at high concentrations (10 microM), while the 7-hydroxy metabolite had no overtly negative effects on proliferation.