IL-27 is a novel heterodimeric cytokine of the IL-12 family that plays an important role in the regulation of T cell responses. Its role on human B cells has not been previously studied. In this study, we show that both chains of the IL-27 receptor complex, IL-27R and gp130, are constitutively expressed at the surface of naive and memory human tonsillar B cells, and are induced on germinal center B cells following CD40 stimulation. In naive B cells, IL-27 induced strong STAT1 and STAT3 phosphorylation, whereas it induced moderate STAT1 and low STAT3 activation in memory B cells. IL-27 induced T-bet expression in naive and memory B cells stimulated by CD40 or surface Ig engagement, but induced significant IL-12Rβ2 surface expression in anti-Ig-stimulated naive B cells only. In anti-Ig-stimulated naive or memory B cells, IL-27 also induced CD54, CD86, and CD95 surface expression. In addition, IL-27 increased proliferation of anti-Ig-activated naive B cells and of anti-CD40-activated naive and germinal center B cells, but not of CD40-activated memory B cells. These data indicate that the B cell response to IL-27 is modulated during B cell differentiation and varies depending on the mode of B cell activation.