The role of suppressor cells and of their precursors was examined in A/J mice, immunized or tolerized-immunized with rabbit gamma globulin. Antibody response and tolerance were assessed by antigen elimination, followed by an indirect plaque-forming assay. Reconstitution experiments were performed to estimate loss of cooperative capacity in thymus and spleen cells. Infectious tolerance was examined by reconstitution with mixtures of spleen or thymus cells of normal and tolerant donors. Infectious tolerance could not be detected after neonatally induced tolerance. It could be detected when tolerance was induced 11-16 days after birth. Under these circumstances, loss of cooperative capacity and increased capacity for infectious tolerance occurred rapidly over the first 2 days and reached completion by the 10th-20th day after administration of tolerogen. Thymectomy, after tolerance induction, resulted in relative recovery of responsiveness of spleen cells and loss of capacity for infectious tolerance. Pretreatment with cyclophosphamide resulted in a less profound state of unresponsiveness and in the disappearance of the capacity for infectious tolerance. Simultaneous treatment with tolerogen and colchicine also resulted in a less profound state of tolerance. This effect of colchicine was more profound when a low dose of tolerogen was used or when animals were thymectomized before administration of tolerogen and colchicine.