Traditionally perceived as a seco-steroid hormone involved in the regulation of calcium metabolism, 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] is now known also to be active in tissues not directly contributing to mineral metabolism. New data show that 1 alpha,25(OH)2D3 is produced by and interacts with hematopoietic cells. The hormone promotes myeloid differentiation and modulates the function of activated lymphocytes. Another new target tissue for 1 alpha,25(OH)2D3 is the skin, where 1 alpha,25(OH)2D3 enhances differentiation of epidermal cells. Therapeutic application of 1 alpha,25(OH)2D3 or suitable analogues in differentiation disorders of hematopoietic and skin cells is currently under investigation.