In the rat combination DA into PVG, liver grafts are not rejected but induce donor-specific transplantation tolerance. We have examined the immunosuppressive properties of spleen cells from PVG recipients of DA liver grafts at various times post-grafting. The results indicate the development of two phases of cell-mediated suppressor activity, which appear to be mediated by separate spleen cell populations. Mitomycin-C-treated spleen cells taken from animals between 5 and 28 days post-grafting were able to suppress rat mixed lymphocyte reactions (MLRs). These 'early' suppressor cells were glass adherent and absent from populations purified by passage through nylon wool or G10 Sephadex columns. Suppression of MLR by purified glass adherent cells was not specific for either stimulator or responder haplotypes and was blocked by indomethacin. Nylon wool purified T cells were not suppressive at this time. Spleen cell suppressor activity declined to background levels after 35 days post-grafting. However, spleen cells from long-term surviving liver graft recipients (20 weeks or more) were again able to suppress MLR; the 'late' suppressor cells were nylon wool non-adherent and suppression was specific for the donor (DA) MHC type. We conclude that liver grafting in this combination generates early and late phases of suppression among spleen cells, that the early phase is produced by macrophages and mediated by prostaglandins and that the late phase is dependent on allospecific suppressor T cells.