Peripheral blood lymphocytes from both normal donors and gastric cancer patients contained suppressor cells which could be activated by concanavalin A (Con A) to suppress the proliferative response of lymphocytes from a normal donor. Con A-activated suppressor cells resided in the E rosette-forming cell fraction. An apparent inverse relationship was found between Con A-activated suppressor cell activity and lymphocyte proliferative response to phytohemagglutinin. Significant increases of suppressor cell activities were found in advanced gastric cancer patients. These activities decreased remarkably after surgical resection of the primary tumors. It is likely that nonspecific suppressor cells represent one of the major factors inducing immunosuppression in gastric cancer patients.