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This experiment was carried out to study the properties of casein micelles obtained from colostral skim milk. As lactation was progressed from parturition until 240h after calving, the content of total protein decreased while the proportion of casein to whey protein increased. Fractionaltion according to the site of casein micelle was done by ultracentrifugation at 100,000 ×g for 10 minutes(pellet 1), 30 minutes(pellet 2) and 60 mintes(pellet 3) and the serum casein was prepared by acid precipitation of final supernatant at pH 4.6. During the lactation period, the relative amount of pellet 1(large size) decreased, that of pellet 2(middle size) maintained nearly constant level except for pllet from parturition, that of pellet 3(small size) increased, and the serum casein showed almost constant level. The relative amounts of α_(s1)-casein and α_(s2)-casein and β-casein-5P in the pellets decreased and that of χ-casein increased markedly with decreasing micelle size, but the relative amounts of β-casein-1P(f 29-209), (f 106-209) and (f 108-209) showed little change. The composition of the serum casein was different from that of the skim milk casein.
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