Ligation of CD40 on dendritic cells (DC) triggers production of IL-12. Using an adoptive transfer model we have previously shown that rIL-12 acts directly on DC to enhance presentation of an otherwise poorly immunogenic tumor peptide. Using the same experimental model, we now describe a similar adjuvanticity of CD40 ligation on peptide presentation by DC. We also explore the possibility that the IL-12 resulting from CD40 ligation directly affects the APC function of DC, mediating or contributing to the adjuvant effect of CD40 ligation. CD40 engagement in vitro and rIL-12 at concentrations in the range induced by CD40 ligation were equally effective in priming DC for presentation of the tumor peptide in vivo. Remarkably, the copresence in vitro of neutralizing Ab to IL-12, but not to TNF-α, IL-1β, or IFN-γ, ablated the enhancing effect of CD40 engagement on the APC function of DC. These data suggest a major role for autocrine IL-12 in DC modulation via CD40 ligation.