Casein micelles of bovine skimmed milk were fractionated by permeation chromatography on porous glass (CPG-10, 50 nm followed by CPG-10, 300 nm) at 30°C. Micelles were pooled in eight eluant fractions and their size distribution was determined by electron microscopy. The composition of casein in the eight fractions was determined by quantitative hydroxyapatite chromatography. Micelle size decreased progressively with increasing elution volume, and volume-to-surface average diameter ranged from 154 nm in fraction 1 to 62 nm in fraction 8. Concurrently there was a decrease in relative proportions of α s - and β-caseins and a large enrichment of κ-casein, which changed from 4.1% total casein in fraction 1 to 12.1% total casein in fraction 8. At least half the decrease in α s -casein proportions was attributed to the α s1 -casein component, but the data also suggested a decline in proportions of α s2 -casein in the smallest micelle fractions. A plot of κ-casein fractional content versus micelle surface-to-volume ratio gave a straight line (correlation coefficient from linear regression 0.98) from which an average κ-casein surface coverage of 1.5 m 2/mg or 47.3 nm 2/molecule was obtained. If a constant surface coverage for κ-casein is assumed, the parameters of the linear equation predict that micelle voluminosity is inversely related to micelle diameter, being approximately 30% larger in fraction 8 compared to fraction 1.