We studied the effects of the gp120 glycoprotein of human immunodeficiency virus type 1 on the expression of interleukin-12 (IL-12) in human monocytes and in monocyte-derived macrophages. Induction of the mRNA for both the p35 and p40 subunits of IL-12 was observed in both cell types after gp120 treatment. We then evaluated cytokine secretion by using an enzyme-linked immunosorbent assay which recognizes only the IL-12 heterodimer. No IL-12 was detected in monocytes/macrophages treated with gp120 alone. A consistent IL-12 secretion was found in macrophages primed with gamma interferon (IFN-gamma) and subsequently treated with gp120. Low levels of IL-12 were occasionally observed in IFN-gamma-primed monocytes stimulated with gp120. The greater response of macrophages than of monocytes to the priming effect of IFN-gamma was consistent with the finding that IFN-gamma induced a much stronger antiviral state to vesicular stomatitis virus in macrophages than in monocytes. These data indicate that gp120 is an inducer of IL-12 expression in monocytes/macrophages and that IFN-gamma is an essential cofactor for IL-12 secretion, especially in differentiated macrophages.