Adherent spleen cells from late (greater than or equal to 18 days post inoculation) tumor-bearing BALB/c mice suppressed lymphoproliferative and effector immunity as evaluated by the mixed leukocyte culture and cell-mediated lympholysis assays. Procedures that eliminated T-cells or B-cells while enriching for macrophage populations significantly augmented the suppression, whereas removal of phagocytic and adherent cells abrogated the suppressive effect. We concluded that the cells responsible for suppression of cell-mediated immune responses in late tumor-bearing mice were of the monocyte-macrophage series. Furthermore, the suppressive influence was not due merely to the increased number of macrophages in tumor-bearing animals. Experiments clearly showed that the splenic macrophages, even at low concentrations, demonstrated a suppressive function.