2,4-Diaminotoluene (DAT) has been demonstrated to be a potent carcinogen. The present studies were carried out to determine the toxic and immunotoxic potential of DAT. Mice exposed to DAT at 25-100 mg/kg per day for 14 days by gavage showed a 42% increase in liver weight and a slight decrease in spleen weight. Histopathologic evaluation of selected organs showed the liver to be the major target with morphological changes which were dose dependent. The high dose (100 mg/kg) was associated with moderate centrilobular necrosis. No abnormal structure was noted in the spleen, lungs, thymus, kidney or mesenteric lymph nodes. The liver toxicity was associated with an elevation in alanine aminotransferase activity. The only change noted in selected hematologic parameters was a 64% increase in peripheral blood leukocytes. Mice exposed to DAT showed a decreased IgM and IgG response to sheep erythrocytes. The decrease was not a function of a decreased number of B cells because the number of B cells increased dose dependently. Proliferative capacity of immunocompetent cells was not impaired by exposure to DAT as measured by the response to several mitogens. The delayed hypersensitivity response to keyhole limpet hemocyanin in mice exposed to DAT was increased. Natural killer cell activity was decreased dose dependently and may represent a spleen cell pool shift because the number of B cells increased in the presence of a decreasing spleen size. Serum C3 was suppressed at the high dose of DAT. Phagocytosis by splenic macrophages, but not peritoneal macrophages, was inhibited by DAT exposure. DAT exposure for 14 days decreased host resistance to the bacteria, Streptococcus pneumoniae and Listeria monocytogenes, while host resistance to the pulmonary tumor model, B16F10, and the PYB6 fibrosarcoma was unaffected by DAT exposure. These data indicate that DAT is hepatotoxic and perturbs the differentiation and maturation of leukocytes.