We have demonstrated that OKT11A, a mouse monoclonal antibody recognizing the E receptor on human T lymphocytes, strongly enhances the autologous mixed lymphocyte culture (AMLC) response induced by T lymphoblasts, while inhibiting the AMLC response induced by B lymphoblasts as well as the T cell response to alloantigens and soluble antigens. This antibody seems to enhance the idiotype-anti-idiotype interaction between resting and activated T lymphocytes.