It was indicated from ultraviolet difference spectra and ultracentrifugal experiments that associations occurred between two casein components (αs and, κ-caseins, β- and κ-caseins and αs- and β-caseins) at lower CaCl2 concentrations (2_??_3mM) and that aromatic amino acid residues participated in the associations. Chemical modification studies with 2-hydroxy-5-nitrobenzylbromide indicated that tryptophane residues of each casein component were not essential for these associations. It was also demonstrated by nitration of tyrosine residues with tetranitromethane that tyrosine residues of k-casein were essential for αs•κ-association and for κ-association and that tyrosine residues of as casein were important to αs•βassociation. Interactions between casein components were also studied at higher CaCl2 concentration (10mM) which is enough for micelle formation. It was found that tyrosine residues of κ-casein played an important role for the stabilization of as and β-caseins. Properties of the nitrated-β-casein were almost the same as that of the native β-casein except the absorption spectrum. _??_s•β-Interaction in the presence of 10mM CaCl2 was investigated by use of the nitrated-β-casein instead of the native β-casein. It was proved that αs-casein was stabilized by the nitrated-β-casein and that precipitation of the nitrated-β-casein increased in the presence of αs casein. The mechanism of interactions between casein components at higher CaCl2 concentration (10mM) are discussed in connection with the associations at lower CaCl2 concentrations (2_??_3mM).