An antiproliferative suppressor lymphokine was produced from rat T-cells specifically in response to the poorly immunogenic syngeneic mammary adenocarcinoma 13762A. The tumor-induced suppressor lymphokine (TISL) was produced late in culture (peak production on Days 4 and 5) and showed strong but selective inhibitory activity on a variety of immune responses. The immune peritoneal exudate cell response to a highly immunogenic clone from the parental tumor (clone 18A) and the concanavalin A-stimulated response of nonimmune spleen cells were inhibited strongly by TISL. In contrast, the immune spleen cell response to 13762A and the lipopolysaccharide response of nonimmume spleen cells were unaffected. Preliminary molecular weight and physicochemical analysis of TISL indicated that the molecule was large (Mr greater than 350,000); partially sensitive to 75 degrees C treatment for 15 min and to pH 2.0 treatment; only partly degraded by the enzymes trypsin, chymotrypsin, and proteinase K; and completely destroyed by boiling. Although TISL was produced specifically in response to 13762A tumor, prior immunization in vivo was not necessary for the induction of the suppressor lymphokine. These results indicate that populations of rat lymphocytes contain naturally occurring TISL secreting cells, which can be activated specifically by tumor antigens such as those expressed by 13762A.