Recovery from specific immunological tolerance to sheep erythrocytes induced with the drug cyclophosphamide was studied with the hemolytic plaque technique of Jerne. The base line plaque (19S antibody-forming cell of the unstimulated spleen) and the proliferative response to antigen, both of which had disappeared during tolerance induction, returned with the recovery of specific immunological reactivity. When cyclophosphamide is injected without sheep cells there is temporary immunological unreactivity and lymphoid depletion of the spleen, but specific tolerance is not induced. Recovery is largely complete at the end of 2 wk and does not require the participation of the thymus. When cyclophosphamide is injected together with sheep cells, 18 days after drug injection, tolerance is still complete. In nonthymectomized mice there is rapid recovery during the next 10 wk, followed by much slower restoration over the remaining 20-30 wk of observation. The entire recovery process evidently takes 40-50 wk. In thymectomized CBA mice only minimal recovery takes place in the first 10 wk and no further restoration occurs thereafter. Thymectomy performed 18 days after tolerance is induced, when tolerance is complete, is equally effective in preventing this recovery.