An efficient Th1-driven adaptive immune response requires activation of the T cell receptor and secretion of the T cell stimulatory cytokine IL-12 by activated antigen-presenting cells. IL-12 triggers Th1 polarization of naive CD4⁺ T cells and secretion of IFN-γ. We describe a new heterodimeric cytokine termed IL-27 that consists of EBI3, an IL-12p40-related protein, and p28, a newly discovered IL-12p35-related polypeptide. IL-27 is an early product of activated antigen-presenting cells and drives rapid clonal expansion of naive but not memory CD4⁺ T cells. It also strongly synergizes with IL-12 to trigger IFN-γ production of naive CD4⁺ T cells. IL-27 mediates its biologic effects through the orphan cytokine receptor WSX-1/TCCR.