Bronchial asthma, characterized by eosinophilic inflammation in airways, may involve Th2-type cytokines such as interleukin-5 (IL-5). IL-12, a newly established cytokine, induces IFN-gamma production, which may have a regulatory effect on the production of Th2-type cytokines. We examined the effects of IL-12 on the productions of IL-5 and granulocyte/macrophage colony-stimulating factor (GM-CSF) by antigen (Dermatophagoides farinae, Df)-stimulated mononuclear cells (MNCs) from asthmatic patients in vitro. IL-12 enhanced IFN-gamma production and inhibited the production of IL-5 but not of GM-CSF by Df-stimulated MNCs from asthmatic patients. Exogenous IFN-gamma directly inhibited IL-5 production by Df-stimulated MNCs and the inhibition of IL-5 production by IL-12 was partially blocked by anti-IFN-gamma antibody in culture, indicating that inhibitory effect of IL-12 on IL-5 production by antigen-stimulated MNCs is partially dependent on IFN-gamma production. IL-12 also inhibited the release of eosinophil survival-stimulating factor from MNCs. These results indicate that IL-12 may be therapeutically beneficial in correcting Th1/Th2 imbalance in bronchial asthma.