Soluble material enriched in tumor-associated antigen was prepared by affinity chromatography from a KCl extract of the chemically-induced D-23 rat hepatoma. Microgram quantities of the above material bound spontaneously to living BCG when the two were incubated briefly in vitro. When injected into normal syngeneic rats, the BCG-associated tumor antigen induced a measure of resistance against challenge with D-23 tumor cells. Peritoneal exudate cells (PEC) obtained from such actively immunized subjects were able to suppress the growth of D-23 tumor cells at a test site in muscle. In contrast, immunization with either BCG alone, tumor protein alone, or tumor protein admixed with BCG in circumstances designed to impede association of the protein, failed to provoke the formation of tumor suppressor PEC. The results encourage of the belief that binding of tumor antigen to BCG favors the induction of a cell-mediated tumor suppressive response.