The role of IL-12 in human peripheral blood B cell responsiveness was examined. To analyze the ability of IL-12 to directly mediate B cell growth and/or differentiation, FACS-purified (> 99% pure) B cells were studied and a polyclonal B cell-activating system utilizing Cowan I Staphylococcus aureus was used. Whereas IL-2 is highly effective in this system in promoting both B cell growth and differentiation, IL-12 was observed only to augment modestly B cell growth and to be ineffective by itself as a B cell differentiation factor for S. aureus-stimulated B cells. However, IL-12 markedly enhanced Ig secretion when added in the presence of IL-2. Moreover, when the ability of IL-12 to augment IL-2-dependent B cell Ig secretion was compared with the ability of several known auxiliary B cell differentiation factors, IL-12 was observed to be the most potent cytokine that could costimulate with IL-2. Analysis of IL-12-stimulated B cell cultures failed to reveal outgrowth of T cells and NK cells. In addition, assessment of IFN-gamma levels in IL-12-driven B cell culture supernatants and analysis of IFN-gamma effects on B cell responses added additional support to the conclusion that IL-12 directly modulates B cell function. Finally, these results suggest that IL-12 is a potent constimulus of B cell differentiation and that the signals conveyed by IL-12 seem to be qualitatively distinct from the differentiative signals delivered by other cytokines such as IL-2.