The rates of milk clotting and of formation of para-/c-casein in milk, colloidal phosphate-free milk and isolated /c-casein by pepsin and by its soluble size-fractionated conjugates with dextran were determined. Milk clotting with all the enzyme derivatives was dependent on the rate of the enzymic phase and required essentially complete /c-casein hydrolysis at 30 °C and throughout the range of pH 5-6-6-7. K-Casein hydrolysis by pepsin at pH 6-6 was fastest in milk and slowest in isolated /c-casein, but the rate decreased as the enzyme size increased, especially with milk. When corrected for the changes in pepsin activity, the rates of /c-casein hydrolysis in all substrates were identical at 30 and 5 °C, but increased with decrease in pH, especially with the larger enzyme conjugates. Hydrolysis of the C-terminal bonds of /?-and /c-casein in native and disrupted casein micelles by carboxypeptidase A and soluble conjugates of it were also investigated. /c-Casein was hydrolysed much faster, and /^-casein slightly faster, in native than in disrupted micelles by the native enzyme. Increase in the size of carboxypeptidase A increased the rate of hydrolysis of /c-casein in disrupted micelles and also induced lag periods before hydrolysis commenced, especially with disrupted micelles. The results are compatible with a model for the casein micelle in which the /c-casein is on the outside and the casein components are in a more ordered arrangement than in the casein complexes formed on micelle disruption. They also indicate that immobilized coagulants would be unable to clot milk.