We have described previously a family in which several members have disseminated Mycobacterium avium complex infection. PBMC from affected members produced abnormally low amounts of IFN-gamma upon stimulation with PHA. Using PHA-stimulated allogeneic cocultures of highly purified monocytes and T cells from familial patients and normal subjects, we have now demonstrated that familial patient monocytes are defective in accessory cell function for IFN-gamma production. Familial patient monocytes did not inhibit IFN-gamma production by normal cells, nor did inhibition of PG synthesis restore normal IFN-gamma production by familial patient cells. Familial patient cells responded to the addition of exogenous IL-12 by increasing IFN-gamma production, while addition of exogenous anti-IL-12 had an insignificant effect on their IFN-gamma production. IL-12 was undetectable in PHA-stimulated cocultures of familial patient monocytes with familial or normal T cells. In addition, IL-12 production by adherent cells from patients and their unaffected mothers was abnormally low following stimulation with fixed Staphylococcus aureus Cowan I strain. However, normal amounts of IL-12 were detected when adherent familial patient cells were stimulated with S. aureus Cowan I strain and IFN-gamma, suggesting abnormal regulation of IL-12 production by familial monocytes. This is the first report of defective IL-12 production associated with increased susceptibility to an infectious disease, a finding that supports the critical role of this cytokine in host defense.