Activated B cells, pulsed with tumor-lysates, were examined for their potential to induce tumor-specific CD4+ T cells in vivo, MH2 cells, which are CD4+ T cells and can also recognize purified protein derivative from mycobacterium tuberculosis (PPD), showed a proliferative response in the presence of both syngeneic activated B cells and PPD in a major histocompatibility complex class II-restricted manner. For B cells to function as efficient antigen presenting cells, they need to be activated. Both irradiation and several inhibitors for antigen-processing were observed to block the antigen-presenting ability of activated B cells. Based in these findings, the possibility of anti-tumor vaccination with activated B cells was thus investigated. The spleen cells from mice, which were immunized with activated B cells pulsed with B16 melanoma-lysates, produced a higher level of interferon (IFN)-gamma than those from mice, which were immunized with either non-pulsed activated B cells or the tumor-lysates alone, after in vitro restimulation. This IFN-gamma production was also dependent on the CD4+ T cells. Moreover, the splenic CD4+ T cells in such mice were suggested to increase their ability to generate B16 melanoma-specific cytotoxic T lymphocytes after in vitro restimulation. Even more importantly, immunization with B16 melanoma lysate-pulsed activated B cells elicited a protective immunity against B16 melanoma at rechallenge. Collectively, these results indicate that an anti-tumor effect could be induced by immunization with activated B cells, pulsed with the tumor-lysates, by eliciting tumor-specific IFN-gamma producing CD4+ T cells in vivo.