Summary A technique is described for the separation by ultracentrifugation of up to 99% casein from milk. The casein gel is transformed by an ion exchange procedure to colloidal alkali caseinate and finally submitted to electrophoretic analysis. This technique guarantees that there is no loss in casein fractions. The level of the residual casein soluble in the ultracentrifugal whey is influenced by the temperature of centrifugation and by added calcium. Temperatures above 20° C. or added Ca up to 6mM per 100ml. of milk before centrifugation improve the casein sedimentation considerably. If no Ca is added to the milk centrifuged at 20 or at 4° C., the amount of soluble casein left is about 6 or 15%, respectively. The soluble casein shows a relatively much higher percentage of β- and γ-casein compared to whole casein.