Undegraded polyribosome preparations may be obtained from chick intestinal mucosa if ribonuclease activity is strictly controlled. This is best achieved by homogenization of the mucosa directly in rat liver cell-sap. 2. The extent of amino acid incorporation by chick intestinal polyribosomes is greatly influenced by the source of the cell-sap. Sephadex-treated intestinal cell-sap caused impaired incorporation and release of completed polypeptide chains, whereas Sephadex-treated rat liver cell-sap promoted the polymerization of up to 90 amino acids per ribosome. Under optimum conditions 30–35% of the nascent polypeptide chains are completed and released. 3. The preparation of an antiserum against the calcium-binding protein formed in response to vitamin D is described. It is shown that the antiserum is highly specific for calcium-binding protein. 4. This antiserum was used to investigate the ability of chick intestinal polyribosomes to synthesize calciumbinding protein. Only polyribosomes from chicks receiving vitamin D have the ability to synthesize calcium-binding protein. Moreover, the product formed in vitro has the same electrophoretic mobility as calcium-binding protein synthesized in vivo. 5. It is concluded that one of the main functions of vitamin D in the small intestine is to induce the synthesis de novo of calcium-binding protein.
