Although transfection to express any of a multitude of immunomodulatory molecules can lead to the rejection of murine tumors in vivo, it is not clear which of these cofactors are truly important for the induction of tumor-specific CTL. Examination of the costimuli used by the host immune response during the normal rejection of immunogenic tumors should reveal critical cofactors for CTL differentiation in vivo. The involvement of a host B7 family costimulator molecule in the rejection of immunogenic tum- variants of the mastocytoma P815 was explored. Rejection of immunogenic P815 variants was prevented by mCTLA4 gamma 3, a fusion protein between the extracellular domain of murine CTLA4 and the Fc portion of a murine IgG3 Ab, indicating the importance of a CTLA4 ligand provided by the host in the rejection of B7- tumors. Tumor rejection also was prevented by mCTLA4 gamma 3 in the absence of CD4+ cells, suggesting that CD8+ lymphocytes may receive direct costimulation by B7 in vivo. Finally, although living transfectants of poorly immunogenic P1.HTR cells expressing B7-1 or B7-2 were equally rejected by syngeneic mice, if delivered as multiple injections of irradiated cells, only B7-1 transfectants successfully induced CTL activity and protected against living tumor challenge. Our results indicate that a CTLA4 ligand is normally involved in the generation of CD8+ CTL against tumor Ag and suggest that immunization with irradiated B7-1-transfected tumor cells may be superior to immunization with irradiated B7-2 transfectants as an approach to tumor Ag vaccination in patients.