Compared to normal hosts, tumor-bearing BALB/c mice were more susceptible to the immunosuppressive effect of tumor cells. At least a tenfold increase was found in the susceptibility mediated by a population of radioresistant spleen adherent cells (AC). The experiments were performed by the study of the suppressive effect of tumor cells on the generation of cytotoxic T-cells in allogeneic mixed lymphocyte culture reactions and allogeneic mixed lymphocyte tumor cell culture reactions. Fewer tumor cells were needed to suppress the T-cell-mediated cytotoxic responses of tumor bearers compared to normal hosts. A normal spleen population could be made to react like the tumor-bearing host by first depletion of its normal macrophages and then reconstitution with spleen AC from tumor bearers. Conversely, reconstitution of the macrophage-depleted tumor bearer's spleen with normal spleen AC made the tumor bearer react like the normal host. Furthermore, tumor cells were needed to trigger the spleen AC of the tumor bearer to fully exert their effect.