We investigated the effects of IL-12 on immunoglobulin (Ig) production in vitro in murine chronic graft-vs. -host disease (cGVHD), a lupus-like model of overt B cell activation induced by allogeneic stimulation. Addition of IL-12 to cGVHD splenocytes strongly inhibited total Ig (Igkappa), IgM and IgG1 production. Although IL-12 down-regulated IL-4, IL-5, IL-9 and IL-10 production, its inhibitory activity on Ig production could not be ascribed to down-regulation of these cytokines, as addition of saturating doses of IL-4, IL-5 and/or IL-9 did not reverse the inhibitory activity of IL-12. Interestingly, IL-12 was also found to suppress the stimulating effect of IL-4 and IL-5 on Ig synthesis by cGVHD splenocytes. Several lines of evidence indicated that the inhibitory activity exerted by IL-12 on Ig production was mediated by IFN-gamma. First, IFN-gamma was produced in large amounts upon IL-12 stimulation. Secondly, it displayed a potent inhibitory activity on Ig production. Thirdly, Ig production was also inhibited by IL-18, a recently cloned IFN-gamma-inducing cytokine. Finally, the inhibitory activity of IL-12 was blocked by anti-IFN-gamma monoclonal antibody. We also investigated whether IL-12 down-regulated Ig production by purified cGVHD B cells. We found that IL-12 had only a marginal inhibitory activity on highly purified B cell populations isolated from cGVHD splenocytes and stimulated with IL-4 and IL-5, and that IL-18 was inactive in this respect. However, when the two cytokines were combined, a striking synergy was unmasked not only for IgG1 inhibition but also for IFN-gamma production by these B cell populations. Taken together, our results demonstrate that IL-12 inhibits in vitro Ig production by activated splenocytes through IFN-gamma production and that it synergizes with IL-18 on activated B cells to inhibit Ig production, through up-regulation of IFN-gamma production by B cells.