Interleukin (IL)-17 is a pro-inflammatory cytokine that is produced by activated T cells. Despite increasing evidence that high levels of IL-17 are associated with several chronic inflammatory diseases including rheumatoid arthritis, psoriasis, and multiple sclerosis, the regulation of its expression is not well characterized. We observe that IL-17 production is increased in response to the recently described cytokine IL-23. We present evidence that murine IL-23, which is produced by activated dendritic cells, acts on memory T cells, resulting in elevated IL-17 secretion. IL-23 also induced expression of the related cytokine IL-17F. IL-23 is a heterodimeric cytokine and shares a subunit, p40, with IL-12. In contrast to IL-23, IL-12 had only marginal effects on IL-17 production. These data suggest that during a secondary immune response, IL-23 can promote an activation state with features distinct from the well characterized Th1 and Th2 profiles.