Tumors engineered to express the costimulatory molecule B7-1 can elicit CD8+ cytotoxic T lymphocyte (CTL)-dependent antitumor responses in immunocompetent mice. It has been postulated that this result reflects direct priming of CTL by the modified tumor in vivo. Previous studies of the immune response to a B7-1- murine colon carcinoma expressing influenza nucleoprotein (NP) as a model tumor antigen have demonstrated the crucial role of bone marrow-derived antigen-presenting cells (APCs) in the priming of NP-specific CTL in vivo. In this system, no evidence of direct CTL priming by tumor was detected. We have performed a similar analysis to determine if B7-1 transfectant of this tumor results in the direct priming of CTL, and to compare this response to that primed by host APCs. When H-2b-->H-2bxd bone marrow chimeras were immunized with a single injection of CT26/NP/B7-1 (H-2d), NP-specific CTL were detected that were restricted to the bone marrow haplotype (H-2b), but not to the tumor haplotype. In contrast, CTL recognizing the NP antigenic epitope in the context of the tumor's major histocompatibility complex were detectable only after multiple immunizations. These results suggest that whereas B7-1+ tumor vaccines result in some degree of direct presentation to CD8+ T cells, the dominant mechanism of CTL priming is through the uptake and presentation of tumor antigens by bone marrow-deprived APCs. However, repeated immunization with B7-1+ tumor cells can efficiently expand the directly primed CD8+ CTL population.