The interaction of κ-casein and β-lactoglobulin is fundamental to all heat-induced modifications of milk product functionality, such as the heat stability of concentrated milks. Purified native κ-casein B and β-lg A solutions were heated at 80 °C at pH 6·7 separately and in a mixture. The circular dichroism spectra in the near UV indicated irreversible changes in the disulphide bonding patterns involving both proteins. Alkaline- and SDS-PAGE of heated samples showed that, in the presence of κ-casein, less β-lg was converted into β-lg polymers and the rate of loss of native β-lg was greater. When κ-casein was added to previously heated β-lg and the mixture was heated, the κ-casein reacted with the heat-induced β-lg polymers more readily than with the β-lg native monomers. The formation of β-lg dimers, trimers etc. was diminished. It was concluded that, when β-lg and κ-casein were heated together, β-lg formed thiol-exposed monomers, which reacted with each other or with the native κ-casein depending on the relative concentrations of β-lg and κ-casein. The products of these reactions included some disulphide-bonded 1[ratio ]1 β-lg[ratio ]κ-casein complexes, some monomer κ-casein and a range of large aggregates held together by either or both disulphide bonds and hydrophobic association.