The BB rat is presently the best available animal model for human insulin dependent diabetes (IDD). Because of the extreme susceptibility of the strain to opportunistic infections and because current studies suggest that they have an autoimmune diathesis, of which IDD is but one result, aspects of the immune system of the BB rat were studied. Severe T lymphopenia was observed in all BB rats, irrespective of sex or the presence of IDD, while numbers of B cells and serum immunoglobulin levels were normal. Both the helper T lymphocyte and cytotoxic/suppressor T lymphocyte subsets, defined by reactions with monoclonal antibodies, were depressed, and an inversion of the helper T cell subset to cytotoxic/suppressor T lymphocyte subset ratio occurred in all BB rats with increasing maturity. Concomitantly, severe impairments of T cell-mediated immune responses were noted. BB rats poorly rejected allografts across both major and minor histocompatibility barriers, and BB splenic or peripheral blood lymphocytes had markedly defective proliferative responses to mitogens and to allogeneic cells in MLC. Irradiated and nonirradiated BB spleen cells did not inhibit WF mitogenic or MLC responses, which suggests that the T cell defect in BB rats is not solely due to increased suppressor activity. Because irradiated WF cells and Con A supernatants did not restore BB proliferative responses, and BB lymphocytes were able to produce IL-2 normally, a reduced ability of BB lymphocytes to respond to helper factors such as IL-2 is suggested. In contrast to T lymphocytes from spleen or peripheral blood, BB thymocytes responded as well as did WF thymocytes to Con A or Con A supernatants. Percentages of T lymphocyte subsets and histology of BB thymuses were also normal when compared to WF thymuses. However, spleens and lymph nodes from BB rats were severely depleted of T lymphocytes, and thymocytotoxic autoantibodies were detected in many BB rat sera. The above findings indicate that BB rats have T lymphocyte immunoincompetence, which appears to be a post-thymic or peripherally acquired maturational defect.