The effect of heat treatment on the profiles of casein micelles obtained at 340 and 280 nm from controlled pore-glass chromatography of milk was studied in a series of unhomogenized whole milk samples which had been heated in a pilot plant at 75, 90, 105, 120, 135 and 150 °C for holding times of 4, 16, 32, 72, 150 and 300 s. A second experiment was carried out in which whole milk was preheated at temperatures between 70 and 110 °C for 13, 60 and 180 s before being treated at 120 °C for 4 s or 150 °C for 16 s. The average micelle size in some of the samples was determined by photon correlation spectroscopy. Profiles were divided into four fractions in which micelle size decreased from fraction 1 to fraction 4. The dimensions of these fractions were expressed in terms of size relative to that of the total profile. Fraction 4 was not affected by any of the heat treatments. Fractions 1–3 were significantly affected only by temperatures above 120 °C. Between 120 and 150 °C fraction 1 increased significantly while fractions 2 and 3 showed a corresponding decrease. Holding time affected fraction 1 only above 120 °C. For fraction 2 there was no effect in < 32 s at temperatures less than 150 °C. Fraction 3 was the most sensitive to changes in holding time, but no effect was observed below 105 °C. Average micelle size measurements indicated that a substantial size increase only occurred in those milks heated at 135 °C for longer than 16 s and in all the samples at 150 °C. To assess whether this increase in micelle size was reversible, samples were dialysed against a 70 mm-NaCl solution containing 10 mm-EDTA and then separated by column chromatography on Sephacryl S-300. The proportion of materials which was undissociated by dialysis remained unchanged for temperatures below 120 °C, but progressively increased with treatments more severe than 120 °C for 16 s. Column profiles were not influenced by variations in the conditions of preheating