The release of 12% (w/v) TCA-soluble N-acetylneuraminic acid from casein at 5 temperatures between 110 and 150°C was determined and showed a Q10 °C about 3; coagulation occurred when about 20% of the κ-casein was hydrolysed. Renneting of milk or colloidal calcium phosphate-free milk rendered the caseinate very heat-labile at its normal pH when more than about 20% of the κ-casein had been hydrolysed. Heat stability at the pH of maximum stability was not significantly decreased until after very prolonged renneting, but stability at pH values alkaline to the minimum was very sensitive to such hydrolysis suggesting that κ-casein is the principal factor responsible for heat stability in that region. Systems which do not have a maximum or minimum in the heat coagulation time–pH profile, i.e. serum protein-free casein micelles in milk diffusate, or milk which had been dialysed against water, were destabilized by renneting throughout the pH range 6·4–7·4.