The production of graft-versus-host (GVH) reactions in (PVGc X Wistar) F1 hybrids by the transfer of PVGc spleen cells resulted in significant resistance of these recipients to a subsequent challenge with the PVGc leukaemia. Protection was markedly dependent on dose and timing of allogeneic cell transfer and was abrogated by irradiation of the cells prior to transfer. GVH activity was shown to be a prerequisite for induction of the protective effect but was equally effective when produced by the transfer of Wistar spleen cells in place of PVGc cells. These points, plus the fact that invitro investigations of possible immune mechanisms failed to demonstrate cytotoxic immunity in treated rats, suggested a nonspecific "bystander" effect as the mechanism of protection. The implications of such a mechanism are discussed
