Calcium ions were infused into the external jugular vein of 2 groups of rachitic chicks. One group had been injected with 5 µg cholecalciferol (vitamin D3), while the other group remained vitamin D-deficient. After approximately 72 h the chickens were killed and intestinal mucosa homogenates were prepared for the measurement of calcium-binding protein (CaBP). The results indicate that CaBP is not biosynthesised in response to raising the extracellular calcium ion concentration, and only appears if the chickens had received cholecalciferol