Antitumor effects of CD40 ligation appear to involve distinct antitumor effector cells in different experimental models. In this study, we tested whether T cells were required for antitumor effects of agonistic anti-CD40 mAb (alphaCD40) against immunogenic versus poorly immunogenic tumors. Treatment of mice bearing poorly immunogenic B16 melanoma and its more immunogenic variant, B16-hsp72.1, with alphaCD40 resulted in a similar level of tumor growth suppression. Depletion of T cells did not reduce the antitumor effects in these 2 tumor models. To generate antitumor T cell responses, C57BL/6 mice were immunized with irradiated B16-hsp72.1. Treatment of these vaccinated mice challenged with a high dose of B16-hsp72.1 tumor cells with alphaCD40 induced tumor growth suppression, which was reduced by T-cell depletion, demonstrating that T cells were involved in the antitumor effect of alphaCD40. However, immunized mice depleted of T cells and treated with alphaCD40 were still able to suppress tumor growth as compared to tumor growth in immunized, T cell-depleted mice not treated with alphaCD40, suggesting that T cells were not required for the antitumor effect of alphaCD40. To confirm a lack of correlation between tumor immunogenicity and T-cell requirement in antitumor effects of CD40 ligation, we found that alphaCD40 induced tumor growth suppression in nude and SCID/beige mice bearing highly immunogenic tumors such as Meth A sarcoma, suggesting that macrophages may play a role. Indeed, both poorly immunogenic and highly immunogenic tumors were sensitive to in vitro growth inhibition by macrophages from alphaCD40-treated mice. Taken together, our results indicate that antitumor effects induced by alphaCD40, even against immunogenic tumors, can be observed in the absence of T cells and may involve macrophages.