1. The sensitivity to calcium of the caseinate-phosphate complex of milk heated at 120°C., increases to an early maximum and thereafter steadily declines, apparently in consequence of parallel changes in the sensitivity of its caseinate constituent.
2. A simultaneously developed capacity of the heated complex to bind additional colloidal phosphate appears to be unrelated to these changes in casein sensitivity.
3. Colloidal phosphate content is confirmed as a factor in the heat coagulation of milk.
4. Heat-developed acidity contributes to the heat coagulation of milk primarily by increasing hydrogen-ion concentration and only slightly, if at all, through release of calcium ions from insoluble combinations.
5. Neither serum proteins nor protein-lactose combinations play any significant part in the heat coagulation of milk.
6. The bearing of these results on the heat coagulation of milk in general and on the operation of certain high-temperature stabilization treatments is discussed.