A casein micelle consists of a core of essentially insoluble «"-and/or /3-caseinates stabilized by a coat layer containing -casein. In a system, cores occur in a variety of sizes, thus producing a micelle distribution. Ultracentrifugation is used to characterize reconstituted micelle systems containing as-and -caseins with respect to the weight fractions of total protein in particles of different size. At final environmental conditions of 37°, pH 6.6, a monovalent ionic strength of 0.05 and 0.012 m calcium chloride, it is concluded that a system is in equilibrium when a particular micelle distribution is present and that the distribution is determined by the system weight ratio of 3-: -casein. (A) Micelle systems develop spontaneously as calcium chloride is increased progressively from zero. Using initial weight ratios, Ri, from 20 to 3, the micelle radius of the maximum weight fraction decreases as Ri is decreased, respectively, from ~425 to ~36 nm. Concomitantly, the concentration of nonmicellar protein (including coat) slowly increases; at 8 mg/ml of «¡,-casein, from 0.15 to 0.4 mg per ml. (B) Colloid particles of Cac V .ia spin micelles of bovine milk are nearly spherical and exist in a distribution of sizes. Average radii, by electron microscopy, range from about 10 to 400 nm, with over 95 % of the micelle mass in particles from 30 to 150 nm (Nitschmann,