The binding potencies of OCT to chick intestinal calcitriol receptor and vitamin DBP were approximately 1/8 and 1/600 of the respective values of calcitriol. OCT is circulating mainly as an intact form bound to chylomicrons and/or lipoproteins. Intravenously injected [3H]-OCT to normal rats was quickly decreased from blood and rapid excretion of OCT as a glucuronate into bile was observed. However, significant amounts of radioactivities were recovered in the intact form in liver and intestine even after 24 h. The separation of calcemic and immune and/or differentiation activities may be derived from the rapid turnover and the nonspecific transporting system.