Cell variants from experimental tumors may lose their tumorigenicity or give rise to tumors that regress after a short period of progression in immunocompetent syngeneic animals. Rejection of these tumor cells is often T-cell-dependent. It has recently been reported that, besides the specific signal delivered through the clonogenic receptor, T-cell activation requires a co-stimulatory signal, delivered through its CD28 receptor by B7-1 and/or B7-2 molecules expressed at the surface of the antigen-presenting cells. CTLA4Ig, a fusion molecule that specifically inhibits B7-1 and B7-2 binding to their receptors of T cells, was used to investigate the role of B7 in the spontaneous regression of the tumors induced in syngeneic rats by REGb cells, a regressor cell line established from a chemically induced colon carcinoma. When rats received either 1 or 3 CTLA4Ig injections, REGb tumors grew 3 or 7 times larger than in control animals, respectively. However, in most animals, single or repeated CTLA4Ig injections delayed rather than suppressed REGb tumor rejection. Antibodies to CTLA4Ig appeared in treated rats and could explain this transient effect. Neither REGb cells nor freshly isolated MHC class-II+ antigen-presenting cells infiltrating REGb tumors expressed B7, establishing that the target of CTLA4Ig was not located inside the tumor. In contrast, MHC class-II+ B7+ accessory cells were found in the tissue, rather than the tumor itself, was the site of tumor-antigen presentation to tumor-specific T cells. These results establish the role of B7/CD28 co-stimulation pathway in the control of a spontaneously regressive tumor.