Expression of B7.1 costimulatory molecules on tumor cells has been shown to elicit antitumor immunity in mice. In the present study, we have developed a human B7.1 retroviral vector system to effectively transduce human melanoma cell lines and investigated the potential role of B7.1 in the generation of tumor-specific CTLs from peripheral blood lymphocytes (PBLs) in vitro. We have demonstrated that B7.1-modified melanoma cells are able to induce primary CTL activity from autologous, human lymphocyte antigen (HLA) class I-matched allogeneic PBLs and purified CD8+ T cells in the absence of exogenous cytokines. CTLs generated by B7.1 are tumor specific and HLA class I restricted, and CD8+ T cells are primarily responsible for this specific cytotoxicity. Furthermore, CTLs generated from HLA class I-matched PBLs by B7.1 are cytolytic to tumor cells autologous to the stimulated PBLs. These data suggest that B7.1-modified tumor cells can be used as a potent tumor vaccine for both autologous and HLA class I-matched allogeneic patients.