Aflatoxin B1 (AFB1), a secondary metabolite of Aspergillus flavus and Aspergillus parasiticus, is known for its potent carcinogenicity and immunosuppressive effects. It is also known that AFB1 toxicity appears in different degrees in different animal species and strains. The present study was performed to reveal the involvement of the hypothalamus-pituitary-adrenal gland (HPA) axis in the immunosuppressive effects of AFB1 on C57B/6 mice. Splenic lymphocy1es were assayed to investigate their phenotyping using flow cy1ometry, proliferative response against mitogen and allogenic lymphocy1es, cy1oly1ic cell activity, and IL-2 production. In addition, antibody-mediated immunocompetence was checked using sheep red blood cell (SRBC)-challenged animals by plaque-forming cell (PFC) assay and enzyme-linked immunosorbent assay (ELISA). Corticotropin releasing factor (CRF) in brain hypothalamus and cerebral cortex, plasma adrenocorticotropic hormone (ACTH), and corticosterone were determined by radioimmunoassay (RIA). Hypothalamic catecholamine and its metabolites were assayed by high-performance liquid chromatography (HPLC). The adrenalectomized animals and their respective control animals were used to evaluate corticosterone involvement in AFB1 immunosuppressive effects. A relatively higher dose was applied in the present study, compared to the previous studies that used different strains of mice. Immunosuppressive effects were observed in blastogenic response, IL-2 production, and primary antibody production of splenic cells. The amount of circulating anti-SRBC antibody was also affected. Decreases were observed in the helper-T cell and B cell percentage in phenotyping splenic lymphocyte. No significant changes were observed in natural killer cell activity, mixed lymphocyte response, brain biogenic amine concentrations, concentration of CRF in the hypothalamus, and those of ACTH and corticosterone in plasma. However, the expected effect of adrenalectomy to compensate for the immunosuppression of AFB1 was not observed. The results indicate that the HPA axis does not appear to have a major role in AFB1-induced immunotoxicity.