This study shows that surgical removal of the meth A fibrosarcoma from its semisyngeneic host fails to result in postexcision immunity to growth of a tumor implant unless the host already has acquired a mechanism of concomitant immunity to growth of an implant. Therefore, tumor excision does not cause immunity to be generated but preserves a mechanism of concomitant immunity that already exists and which otherwise would eventually undergo down-regulation under the influence of suppressor T cells. Removal of the tumor after it has grown large enough to cause the T cell-mediated suppression of concomitant immunity does not result in the reemergence of immunity. Instead, the host remains unable to generate concomitant immunity to a second tumor for a long period of time and retains, for at least 31 d, suppressor T cells able to passively transfer suppression to appropriate recipients. Like the suppressor T cells responsible for active suppression of concomitant immunity, the suppressor T cells responsible for "memory" suppression are of the Ly-1+2- phenotype. The results indicate that progressive tumor growth results in a state of immunological tolerance of tumor-specific, transplantation antigens that can persist in the apparent absence of tumor antigens.