Human blood monocytes cultured in the presence of 1,25(OH)2D3 developed enhanced competence for secretion of H2O2 relative to cells suspended in media. This effect was maximal at a concentration of 10(-8) M 1,25(OH)2D3. After 3 days of incubation, monocyte-derived macrophages (MDM) exposed to 1,25(OH)2D3 demonstrated competence for secretion of H2O2 equivalent to cells exposed to recombinant IFN-gamma. Both IFN-gamma and 1,25(OH)2D3 offset decay of this function among cells in culture after 7 days. Simultaneous exposure of cells to 1,25(OH)2D3 and IFN-gamma did not activate competence for H2O2 secretion more than either agent alone. 24,25(OH)2D3 and 25(OH)2D3 activated MDM but at higher concentration than required for 1,25(OH)2D3. Progesterone did not affect H2O2 production. Incubation of MDM with a monoclonal antibody directed against IFN-gamma inhibited activation induced by lymphokine, and to a lesser extent by cells activated with IFN-gamma; this antibody had an insignificant effect on cells treated with 1,25(OH)2D3. These results suggest that 1,25(OH)2D3 exerts a receptor-mediated effect on monocyte function that results in cellular activation as manifested by enhanced competence for secretion of H2O2. It is possible that smaller concentrations of 1,25(OH)2D3 present in serum are permissive for macrophage activation, or that monocytic phagocytes are exposed to high concentrations of vitamin D metabolites under some clinical circumstances.