A synthetic physiologically-active analog of vitamin D, specifically the vitamin D3 form. Calcitriol regulates calcium in vivo by promoting absorption in the intestine, reabsorption in the kidneys, and, along with parathyroid hormone, regulation of bone growth. A calcitriol receptor-binding protein appears to exist in the mucosa of human intestine. Calcitriol also induces cell cycle arrest at G0/G1 phase of the cell cycle, cell differentiation, and apoptosis, resulting in inhibition of proliferation of some tumor cell types. This agent may be chemopreventive for colon and prostate cancers. (NCI04)