T cells are effectively stimulated byDR‐antigens on the surface of autologous non‐T cells. This phenomenon, referred to as an autologous mixed lymphocyte reaction (AMLR), is reported to be impaired in several auto‐immune diseases. In the present study, both AMLR and allogeneic mixed lymphocyte reaction (allo‐MLR) activities of peripheral blood mononuclear cells were examined from patients with auto‐immune chronic active hepatitis. AMLR activity in these patients was much lower than that of controls, while allo‐MLR activity of the same patients revealed no abnormality with regard to either stimulator or responder activities in comparison to controls. Furthermore, co‐culture experiments using cells from patients and their HLA‐identical siblings suggested that the defect resided in the responder T cells rather than in the stimula