Co-culture of autologous T and mitomycin-C treated B cells results in increased DNA synthesis in the responding T cells. T cells thus activated in AMLC exerted suppressive effects on both the proliferative and cytotoxic responses of fresh unstimulated T cells to allogeneic cells in MLC. The suppressor cells generated are sensitive to treatment with mitomycin-C. AMLC-activated cells treated with mitomycin-C failed to suppress both cytotoxicity and proliferation in fresh primary MLC. It appears that the AMLC reaction reflects an immunologic homeostatic mechanism. Since this reaction is defective in patients with CLL and SLE and the homologous mouse syngeneic MLC is defective in NZB mice, the failure of this T-B interaction may be related to the pathogenesis of certain lymphoproliferative and autoimmune disorders.