PURPOSE: To separate a whole casein into a fraction composed of κ-casein and a faction composed of αs-and β-caseins, easily by dissolving the whole casein in a deionized water or in a specific buffer solution, and subjecting the solution to gel-filtration or ultrafiltration.

CONSTITUTION: Whole casein obtained by adjusting an acidic casein to about 7 pH and dissolving again, is dissolved in deionized water or a buffer solution having a univalent salt concentration of ≤0.02 and free from bivalent alkaline earth metal salt (e.g. imidazole buffer solution) at a concentration of 1W30%. The solution is treated at 0W10°C with a gel-filtration or an ultrafiltration device capable of fractionating a component having a molecular weight of ≥100,000 to obtain a fraction composed mainly of κ-casein and a fraction composed mainly of αs-and β-caseins.