AbstractThe lectins phytohemagglutinin, pokeweed mitogen and concanavalin A used at their optimal mitogenic concentration, or human lymphocytes activated by the same mitogens, were found to suppress thein vitrogeneration of cytotoxic effectors when added to a cell‐mediated lympholysis (CML) mixture during the first 48 h of culture. The data suggest that the suppressive mechanism is mediated to a greater extent by an allogeneic interaction between lectin‐activated cells and the allogeneic cells present in the CML mixture than by suppressor cells induced by the lectin. Since partial suppression was observed with supernatants of activated lymphocytes cultured for 18 h with allogeneic stimulating cells (but not activated lymphocytes alone), a soluble mediator may be involved in the suppressive mechanism. The mechanism of suppression therefore may be identical to the preemption phenomenon recently described in primary and secondary