Activity of lysosomal hydrolases cathepsins B, C, D and acid phosphatase was studied in liver tissue and blood serum of rats, ration of which contained 18% and 36% of the sunflower protein concentrate within one and three months. The control animals were maintained on rations with corresponding amount of casein. Activity of the proteinases studied tended to decrease in liver tissue of experimental animals, whereas activity of cathepsins C and D differed from the initial level after 3 months on the experimental diet. These alterations were dissimilar in blood serum: activity of cathepsin C was decreased within 1 month, while the enzyme activity as well as that of acid phosphatase was increased to the end of the experimental period (3 months). The data obtained suggest that lysosomal enzymes are sensitive to alterations in qualitative composition of the ration proteins; this property of the enzymes may serve as a criterion in medico-biological evaluation of new proteins in foodstuffs.