This study investigated the regulatory activity of 1,25-dihydroxyvitamin D-3 (1,25-[OH]D-2(3)) on phagocytic cells obtained from normal human peripheral blood, Flow cytometric analysis enabled identification of two discrete populations of cells, one predominantly monocytes (''monocyte'' gate) and one containing primarily lymphoid and other cell types (''lymphoid'' gate). The monocyte-associated antigens CD13 and CD33 were highly expressed by cells in this monocyte gate and used to monitor this population. Following 5 days of culture, cells in the monocyte gate manifested high phagocytic activity as determined by ingestion of fluorescent carboxyl microspheres and exhibited high expression of class II HLA-DR products. 1,25-(OH)(2)D-3 profoundly upregulated phagocytic activity while downregulating HLA-DR antigen expression on the cells in the monocyte gate. Moreover, 1,25-(OH)(2)D-3 also reduced cell surface CD13 expression on the cells with low but not high phagocytic activity In this gate, Proportional activities by the 1,24-(OH)(2)D-3 and 24,25-(OH)(2)D-3 metabolites indicated the regulatory effects are likely mediated by the 1,25-(OH)(2)D-3 receptor (VDR). Prostaglandin E(2) (PGE(2)), a known modulator of monocyte/macrophage activity also markedly inhibited HLA-DR expression while enhancing the phagocytic activity of cells in the monocyte gate. In contrast to 1,25-(OH)(2)D-3, PGE(2) clearly upregulated CD13 expression in cells with high phagocytic activity. Since indomethacin, an inhibitor of PGE(2) synthesis, failed to reverse the 1,25-(OH)(2)D-3 induced inhibitory effect on HLA-DR expression, this effect is apparently not mediated through endogenous PGE(2) synthesis. Based on these findings we speculate that 1,25-(OH)(2)D-3 may be capable of acting as both an upregulating agent during natural immunity via the enhancement of phagocytosis by monocyte/macrophage populations and as a ''downregulator'' during acquired immune responses via an inhibitory effect on MHC class II antigen expression by professional antigen-presenting cells.