The rate of protein enzymatic degradation in a mixture is slower than that of single proteins. In a mixture of haemoglobin or casein with protamine, the release of tyrosine by trypsin and chymotrypsin A is slower. A slower rate in the release of arginine from protamine occurs only in mixtures with haemoglobin. The inhibition of enzymatic degradation of proteins in a mixture is due to formation of intermolecular associations and to changes in their spacial structure.