MANY of the properties of milk depend on the existence of large aggregates of casein1, the ‘molecular’ weights of which are in the region of 108. When calcium-binding substances such as citrate or oxalate are added, these aggregates are rapidly reduced in size2 to a ‘molecular’ weight of about 105, the specific value being highly dependent on pH, temperature and ionic strength3. The mechanism whereby calcium effects the formation of the aggregates is unknown, but it has often been assumed that the first step is combination with the phosphate groups of casein, and it has been suggested4 that the calcium ions might cross-link adjacent casein molecules. In recent years it has been found that various phosphatases will catalyse the removal of phosphate groups from casein under mild conditions of pH and temperature5,6. Thus enzymatic dephosphorylation should be a useful tool in studying the role of casein-bound phosphate in the reactions of milk dependent on calcium. The clotting of milk by the enzyme rennin is dependent on calcium1, and is a reaction which lends itself to precise study.