Participation of the mononuclear phagocyte system in defense against the emergence and spread of cancer depends upon functional activation of its elements, which requires the acquisition of requisite cellular and biochemical capacities necessary for the execution of the tumoricidal function. Macrophage tumoricidal activation is acquired in discrete stages induced by sequential exposure to specific signals present in the extracellular environment. Macrophage activating factor or interferon-gamma (IFN-gamma) primes the macrophage for a subsequent response triggered by a second signal such as lipopolysaccharide (LPS). The biochemical mechanisms responsible for transducing these signals represent potential regulatory points for therapeutic manipulation of macrophage antitumor function. However, these intracellular events remain largely unidentified. Treatment of responsive macrophage populations with physiologic doses of IFN-gamma for as little as 2 to 4 hours resulted in a five-fold elevation in the total cellular activity of the phospholipid, Ca++-dependent protein kinase [protein kinase c (PKc)]. Furthermore, LPS treatment triggers a series of unique protein phosphorylation events in macrophages. These same phosphorylations can be produced by stimulation with active phorbol diesters suggesting that the protein kinase activity responsible for these specific changes is PKc. Both these findings lead to the hypothesis that IFN-gamma primes macrophages by elevating PKc activity and that LPS triggers stimulation of this enzyme, thus resulting in full activation. This hypothesis will be tested by: (1) identifying the structural and functional basis of the elevation in PKc activity induced by treatment with interferon; (2) determining the molecular basis whereby LPS stimulates the function of PKc; and (3) characterizing the specific protein phosphorylation events induced by both IFN-gamma and LPS. (MB)