Competitive protein-binding assay for 25-hydroxycholecalciferol.

1. A competitive protein-binding assay for plasma 25-hydroxycholecalciferol was developed, using Sephadex LH-20 columns for the isolation of the sterol and a partially purified protein from rat serum as the binding protein. 2. The mean plasma 25-hydroxycholecalciferol in eighteen male and female normal volunteers was 38 5 SD 14 pmol/ml (1 5.2 & 5.6 nglml). 3. Levels of 25-hydroxycholecalciferol in plasma of four male volunteers maintained for 2 months on a diet supplemented with 26-1 nmol (10 pg) of cholecalciferol a day were found to be significantly higher. 4. High correlation was established between plasma 25-hydroxycholecalciferol values obtained by the competitive protein-binding assay and values obtained by bioassay.