We have employed the plaque forming cell (PFC) response to sheep erythrocytes as well as lymphocyte proliferation to study the induction of immunotoxicity in AHH-inducible (Ah Locus positive, C57BL/6N; B6C3F1) and AHH non-inducible (Ah Locus negative, DBA2/N) mice following administration of polycyclic aromatic hydrocarbons. When two potent carcinogenic polycyclic hydrocarbons which induce AHH activity, 3-methylcholanthrene (MCA) or 1,2,5,6-dibenzanthracene [DB(a,h)A] were administered IP, immunotoxicity was observed in both AHH-inducible and AHH non-inducible animals. However, the AHH-inducible animals appeared to be more sensitive, and substantial suppression of a PFC response toxicity could be induced with doses as low as 14 mg/kg methylcholanthrene. While suppression of a mitogen response required a dose of 43-125 mg/kg. Administration of the weak carcinogen 1,2,3,4-dibenzanthracene [DB(a,c)A], IP, which similarly induces AHH activity in inducible animals, failed to induce immunotoxicity in either C57B1/6N or DBA/2N mice. In contrast to the results obtained following IP administration, when MCA was administered repeatedly (4X) via an intragastric (IG) route we observed striking immunosuppression of a PFC response in Ah locus negative (DBA/2) animals but minimal effects in Ah locus positive animals (C57B1/6). We finally observed that a single IP dose of MCA (125 mg/kg) to Ah locus positive animals substantially inhibited Natural Killer Cell activity but had more limited effects on the ability of an animal to reject a challenge by an immunogenic syngeneic fibrosarcoma.