Studies were conducted to determine the ways in which antibody regulates proliferation of lymphocytes stimulated by phytohemagglutinin (PHA) and concanavalin A (ConA). Equine lymphocytes were reacted with PHA or ConA for 1 to 10 hours, washed free of excess lectin, then cultured for 3 days and examined for proliferation and incorporation of [3H]thymidine. Both PHA and ConA induced lymphocytes to proliferate after short incubation periods. This response could be interrupted by the addition of antibody to the respective lectin. Total suppression of the proliferative response occurred if antibody was added within the first 2 hours after initial contact with PHA or ConA. Addition at times up to 48 hours produced either total or partial suppression. The removal of excess antibody 4 hours after addition to the culture did not reverse the suppression. Using radiolabeled ConA and PHA, it was observed that suppressive antibody inhibited the release of lectin from metabolically active lymphocytes and from surfaces where pinocytosis was not possible or was minimized. Antibody did not affect the rate of degradation of lectin by the cells. These results indicate that antibody attached to lectins, already bound to the lymphocyte surface, can terminate the lymphocyte response and retard the rate of release of lectin from the cell membrane.