The cell-mediated immunological response to a rat gliosarcoma was studied. A cytotoxicity assay, in which tumor cells prelabeled with 3H-thymidine were challenged in vitro with immune spleen lymphocytes and PEC, was used. Incubation for 72 h, released significantly higher levels of tritium label from tumor cell-immune spleen cell co-cultures than that released from tumor cells alone or with control lymphocytes. PEC were similarly cytotoxic. A cytostatic assay, where tumor cells were incubated with immune spleen cells and 3H-thymidine, showed significant reduction of incorporation of the labeled precursors into the DNA of the tumor cells compared to that of the controls. There was no significant difference in the inhibitory activity of PEC between the controls and immune animals, except when rats were further immunized by intraperitoneal injection of tumor cells. Incubation of the gliosarcoma cells with the cell-free supernatants from lymphocyte-tumor cell cultures showed that spleen lymphocytes and, to a lesser extent PEC, release in the presence of tumor cells a soluble inhibitory factor, or lymphotoxin. Supernatants of spleen cell-tumor cell co-cultures had an inhibitors effect comparable in terms of cell numbers to that of the spleen lymphocytes when these were directly incubated with tumor cells. This was not the case for PEC, whose supernatants were not effective. Dilution experiments with spleen lymphocytes and PEC, in which the volume of the incubation mixture was increased to 2 and 3 times the original value, keeping the concentration of the labeled thymidine constant, indicate that a major portion of the inhibition by spleen lymphocytes is attributable to a soluble inhibitory factor. On the other hand, the PEC inhibitory activity appears to depend more on a direct interaction with the target cells.