A rapid procedure for the determination of both free and total cholesterol in serum is described. The color development is extremely rapid, and once fully developed the color remains stable for several hours. The heat of reaction produced by the combination of concentrated sulfuric acid, phosphoricacid, and glacial acetic acid is apparently responsible for the rapid color development. The stable ferric chloride reagent containingphosphoric acid appears to be the contributing factor to the stability of the color. The results obtained with this procedure compare favorably with those obtained with saponification methods.