A functionally suppressive antigen-driven subpopulation of macrophages has been demonstrated in mice-bearing tumors produced by MSV-transformed BALB/3T3 cells. The suppressor macrophages depressed markedly both allogeneic and syngeneic mixed leukocyte cultre responses and functioned even when assayed at low concentrations, suggesting that high concentrations of normal macrophages were not responsible for the suppression. Studies also showed that suppressor macrophages acted to suppress the proliferation of hyperreactive Lyt 1+,2−cells found in the splenic cell suspensions of tumor-bearing mice after the removal of the adherent suppressor cells. Finally, the suppressive activity of macrophages induced in tumorous mice could be augmented by KiMSV or MMSV but not by MuLV antigen. These results suggest that suppression might be a direct response to transformation-associated determinants expressed on the tumor cells as well as on the MSV virions or the result of cellular interactions between suppressor macrophages and other antigen-specific spleen cell populations.