Abstract Tyrosyl residues in κ- and (αs1-casein were nitrated with tetranitromethane and its effect on their properties determined. Nitration of up to four of the eight tyrosines in κ-casein did not affect its (αs1-casein-stabilizing ability; additional nitration progressively decreased the stabilizing ability and this was totally absent after all eight tyrosyl residues were nitrated. Nitrated (αs1-casein retained its calcium sensitivity but was unable to form a protective complex with κ-casein after nitration of four of its 12 tyrosyl residues. A complete tyrosyl nitration of κ- and (αs1-caseins and of a 1:10 κ-/(αs1-complex was achieved at low molar ratios of tetranitromethane. This suggests that these proteins are devoid of major secondary or tertiary structures.