Skim-milk was fractionated by permeation chromatography on CPG-10 (300 nm) at 20 °C with synthetic milk serum. The elution profile, which was highly reproducible, showed 3 partly resolved peaks. On treatment with 0.1% glutaralde-hyde before chromatography, a similar profile was obtained indicating no appreciable micellar dissociation during chromatography of the milk. The casein composition of the eluent fractions, determined by quantitative chromatography on hydroxyapatite, showed that 12.1% of total casein was associated with the low molecular weight peak. The micelle content of ²- and κ-casein increased with increasing elution volume, and the ±s/κ ratio showed a progressive diminution from 4·3 to 2·3. Large differences were found in the colloidal phosphate contents and the weight average diameters of the micelle fractions which clearly show that CPG-10 fractionates on the basis of molecular size.