By releasing interleukin (IL)-12 in the lung, alveolar macrophages (AM) may profoundly modify an immune response. The autocrine regulation of the heterodimeric, biologically active form of IL-12 (IL-12 p70) by IL-10 was studied, as well as the expression of its subunits of 35 kD (p35) and 40 kD (p40). AM cultured in medium alone expressed only p35 mRNA. Both p35 and p40 mRNA levels were induced by lipopolysaccharide (LPS) and were further increased by interferon-gamma (IFN-gamma). LPS alone induced IL-12 p40 but not IL-12 p70 production in monocytes and in AM. However, IL-12 p70 was released when the autocrine production of IL-10 was neutralized by IL-10 blocking antibody, and IL-12 p40 production increased. Although IFN-gamma markedly decreased LPS-induced IL-10 production in AM, neutralizing IL-10 further enhanced the level of LPS and IFN-gamma-induced IL-12 p70 in AM. In contrast, neutralizing the trace amount of IL-10 released by AM stimulated by CD40 crosslinking and IFN-gamma did not increase IL-12 p70. Thus, IL-12 p70 production by AM appears to be tightly controlled by the autocrine release of IL-10 when stimulated by LPS, or by LPS and IFN-gamma, whereas CD40 crosslinking triggered IL-12 p70 production in the absence of autocrine regulation by IL-10.