Lymphocytes from regional lymph nodes that drain tumors and extracts of such lymphocytes suppress antibody-dependent cell-mediated target cell lysis by effector cells from cancer patients and controls. In contrast, lymph node cells from control patients are not suppressive. Control extracts from normal lymphocytes and 2 immunologically inert tissues, muscle and kidney, exhibit some suppressive activity but significantly less than extracts from lymph nodes that drain tumors.