1 alpha,25-Dihydroxyvitamin D3, the active form of vitamin D3, induced maturation of circulating monocytes to form macrophage-epithelioid cells and multinucleated giant cells in vitro. Calcitriol not only promoted the differentiation of monocytes, as shown by the marked morphological changes and enhanced secretion of lysozyme, but also induced their prominent proliferation, as exhibited by enhanced DNA synthesis and the increased number of monocyte cell nuclei. The proliferation of monocytes was observed after the addition of physiological concentrations of calcitriol. Multinucleated giant cells were frequently observed among the monocytes. These marked morphological changes and the proliferation of monocytes were not observed in control cultures, which did not include calcitriol. These results indicate that calcitriol plays a critical role in the formation of the sarcoid granuloma and give an explanation of some of the clinical findings on sarcoidosis. In view of the evidence that sarcoid macrophages convert 25(OH)D3 to calcitriol, our results raise the possibility that the active metabolite of vitamin D3, which may be produced by macrophage-epithelioid cells, induces the differentiation and proliferation of circulating monocytes into macrophage-epithelioid cells, which in turn form sarcoidosis granulomas. This autostimulation mechanism of sarcoid granuloma formation may provide a model for future studies.