Many investigations on casein micelles have offered some micelle models. These models are different from one another.') It was reported recently that removal of calcium ion by dialysis2) or reaction with complexing agents',') leads to micelle dissociation, and then subunits were observed to be about 10 nm in diameter by means of freeze-etch electron microscopy by Schmidt and Buchheim.2) Creamer and Berry5) suggested that the subunit is in equilibrium with its component caseins and that its size distributions are changed under various conditions of pH, temperature, and etc. Furthermore, Schultz and Bloomfield6) reported the isolation of oligomer (about 10 nm) from casein micelles (prepared at 5°C) by using of Biogel A-15 column. Although these results indicated that casein micelle consisted of subunit and so on, it is not clear how to assemble subunit in casein micelle because casein micelles for sample are prepared at 5°C and are various sizes. Therefore, in order to estimate the construction of casein micelle, the dissociation process of micelle was studied by using casein micelles in uniform size prepared at room temperature. Skimmilk of less than 0.01 % of fat content was prepared from bulk milk of Holstein cows, and used for experiments under the condition of previous papers.4 Ultrafiltrate of skimmilk was prepared using a ultrafiltration cell (Toyokagaku model UH-76) with membranefilter UK200 (cut-off molecular weight 200,000) at room temperature. Casein micelle in uniform size was prepared from skimmilk. Insoluble substance in skimmilk was removed by centrifugation at 1000 x g for 30 min and the supernatant was centri fuged at 15,000 x g at 20°C for 30 min. The pellet was dispersed into the ultrafiltrate of skimmilk by a Potter-Elvehjem homogenizer, and centrifuged at 15,000 x g for 30 min. Repeating this procedure twice, the micelle in uniform size (above 100 nm in diameter) was prepared. This pellet was dispersed to 3 % of the protein concentration in 0.01 m imidazole buffer (pH 6.8, ionic strength 0.07) containing 0.02 of NaN3. Proteolysis was inhibited by addition of trypsin inhibitor (Miles Labo. Ltd.) to this solution.') Each 20 ml of casein micelle solution was rigidly packed in visking tube, and dialyzed against 0.01 M