Partial suppression of antibody formation in neonatally induced tolerance to HSA as observed during immunization at 6 weeks of age or at a later time, is caused mainly by a rapid escape from tolerance. The following findings support this view: 1. After immunization at 4 weeks of age with a larger dose of HSA, which elicits detectable circulating antibody formation in all control birds, anti-HSA antibodies cannot be demonstrated in a large majority of tolerant chickens. 2. Surgical bursectomy performed in tolerant birds on the day of hatching or 7 or 11 days after hatching increases considerably the degree of suppression of anti-HSA antibody formation and slows down the return of reactivity to HSA. These findings suggest that new immunocompetent cells originating in the bursa are responsible for escape from tolerance in this experimental system. The existence of suppressor cells in birds tolerant to HSA was not demonstrated by the transfers of spleen cell mixtures from immune and tolerant donors to young, irradiated recipients.