The polycyclic aromatic hydrocarbon, 7,12-dimethylbenz[a]anthracene (DMBA), is an immunosuppressor as well as a potent organ-specific carcinogen. To understand the organ-specific mechanism of DMBA-induced lymphoid toxicity, aryl hydrocarbon-nonresponsive mice and microsomal epoxide hydrolase (mEH)-null mice were analyzed. DMBA caused a dose-dependent decrease in spleen weights, but not the thymus weights in aryl hydrocarbon-nonresponsive mice. On the other hand, both spleen and thymus weights were decreased to less than a half in wild-type mice exposed to 30 mg/kg of DMBA. In contrast, no decrease was detected in spleen weights of mEH-null mice exposed to up to 100 mg/kg of DMBA, while thymus weights were markedly lower. Responses to the B-cell mitogen lipopolysaccharide and to T-cell mitogen phytohemagglutinin were nearly completely abolished in splenocytes isolated from wild-type mice treated with 100 mg/kg of DMBA. These responses were decreased, but maintained in splenocytes isolated from mEH-null mice treated with DMBA. Two DMBA metabolites dependent on mEH including DMBA-3,4-diol were detected in an HPLC chromatogram of spleen microsomes isolated from wild-type mice, but not those from mEH-null mice. These results suggest the involvement of mEH in splenic activation of DMBA for immunotoxicity and the difference for the DMBA-induced lymphoid toxicity between spleen and thymus.