1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3), the biologically active form of vitamin D3, has been shown to inhibit proliferation and promote monocytic differentiation of leukemic cell lines. In the present communication, we extend these observations to normal bone marrow macrophage precursors, and 1) identify the stage of monocytic maturation wherein the steroid exerts its antiproliferative effect, and 2) demonstrate that 1,25-(OH)2D3 promotes bone marrow macrophage differentiation as manifest by specific up-regulation of the lineage-specific membrane protein, the mannose-fucose receptor. In these experiments, the 1,25-(OH)2D3-mediated inhibitory effect on colony formation was shown to be independent of attendant levels of colony stimulating factor-1 and targeted through the adherent bone marrow macrophage precursor. Examination of this steroid-sensitive adherent precursor population demonstrates that its specific binding of 125I-mannose bovine serum albumin spontaneously and progressively increases with time in culture. Whereas adherent bone marrow macrophages cultured for 2 days express 3 X 10(4) mannose receptors/cell, the number of binding sites increases to 7 X 10(4)/cell by day 4. When bone marrow macrophage precursors are exposed to 1,25-(OH)2D3, an additional stepwise enhancement of 125I-mannose bovine serum albumin obtains with time. Four days of culture with the steroid results in 1.6 X 10(5) mannose receptors/cell, a 100% increase as compared to control cells. Neither duration of culture nor exposure to 1,25-(OH)2D3 alters the KD of 125I-mannose bovine serum albumin which approximates 3-5 X 10(-9) ml-1. Finally, the "specificity" of vitamin D-mediated up-regulation of the mannose receptor was established by demonstrating that the steroid does not alter binding of 125I-alpha-thrombin by bone marrow-derived macrophage precursors.