A role has been sought for the calcium binding protein (CaBP) which is synthesised de novo after giving cholecalciferol (CC, vitamin D3) to rachitic chicks. After homogenation of mucosal cells in sucrose media, the CaBP was found in the 78,000 X g supernatant. Therefore, the CaBP is either present in the cytoplasm or in some labile membrane structure, e.g. the microvilli, that is disrupted by homogenation. This intracellular CaBP may facilitate diffusion of Ca into intestinal cells. No secretion of CaBP into the lumen could be detected nor did excess CaBP placed in the lumen increase Ca absorption of rachitic chicks. The mitochondria of duodenal mucosal cells contained most of the Ca being translocated by the small intestine. CaBP caused release of Ca already present in mitochondria and diminished Ca uptake by mitochondria and it appreared to do this by increasing the rate of Ca flux across the mitochondrial membrane. This would explain the greater "turnover" of Ca in mucosal cells of cholecalciferol-treated chicks. These and previous findings have been used to propose a scheme for the effect of cholecalciferol on Ca transport from the small intestine.