IL-27 is a member of the IL-6/IL-12 family and activates both STAT1 and STAT3 through its receptor, which consists of WSX-1 and gp130. We previously demonstrated that IL-27 has potent antitumor activities, which are mediated through CD8(+) T cells, NK cells, or its own antiangiogenic activity. In this study, we demonstrate that IL-27 also possesses a direct antiproliferative activity on melanoma. Although WSX-1 expression was hardly detected in parental mouse melanoma B16F10 cells, IL-27 activated STAT1 and STAT3 and up-regulated MHC class I in B16F10 transfectants expressing wild-type WSX-1. In contrast, IL-27 failed to activate STAT1 and up-regulate MHC class I in those expressing mutant WSX-1, in which the putative STAT1-binding Tyr-609 of the cytoplasmic region was replaced by Phe. IL-27 inhibited the tumor growth of transfectants expressing wild-type WSX-1 in a dose-dependent manner. IL-27 augmented the expression of IFN regulatory factor (IRF)-1 and IRF-8, which possess tumor suppressor activities, in B16F10 transfectants expressing wild-type WSX-1. Down-regulation of IRF-1 but not IRF-8 with small interfering RNA partially blocked the IL-27-induced growth inhibition. A small, but significant, direct antiproliferative effect of IL-27 was also observed in vivo. Moreover, several human melanoma cells were revealed to express both IL-27 receptor subunits, and activation of STAT1 and STAT3 and growth inhibition by IL-27 were detected. These results suggest that IL-27 has an antiproliferative activity on melanomas through WSX-1/STAT1 signaling. Thus, IL-27 may be an attractive candidate as an antitumor agent applicable to cancer immunotherapy.