Summary A rapid and simple method for determining the undenatured serum protein in milk, based on measurement of the volume of precipitated serum protein, is described. It involves the acid precipitation of the casein and the denatured serum protein and their removal by filtration. The soluble undenatured whey proteins in 7ml. of filtrate are precipitated with 3ml. of 10% phosphotungstic acid. Upon controlled centrifugation, the volume of the precipitate is obtained and converted to milligrams of whey protein nitrogen per gram of nonfat milk solids by means of a standard curve or the volume of precipitate may be used directly as the unit of measurement. Thus, the suitability of milk for a specific purpose is indicated. The relation between milliliters of precipitate and milligrams of undenatured whey protein nitrogen as determined by Kjeldahl analysis has a correlation coefficient of 0.97. A standard curve is given which can be used routinely with the indicated equipment.