Mice given 7, 12-dimethylbenz[a]anthracene (DMBA) simultaneously with immunization with bacterial α-amylase (BαA) showed an augmented anti-BαA response in sera as compared with those of mice given only BαA, but the development of immunological memory in DMBA-treated mice was constantly and persistently depressed. Administration of syngeneic normal thymocytes to DMBA-treated mice further strengthened the augmenting effect of DMBA on the serum antibody titers, whereas administration of thymocytes to normal mice not treated with DMBA revealed rather suppressive effect on the serum antibody titers. It was also found that most of mice given both DMBA and normal thymocytes suffered from a variety of wasting symptoms resembling those of runt disease or graft-versus-host reaction, and many of them died within 4 to 5 weeks. Histological examination showed almost complete loss of follicular organization in spleen and lymph nodes. A similar but slight histological change was observed in mice given only DMBA. These findings suggest that the immune system of mice was markedly disturbed by the administration of DMBA and normal thymocytes.