Data on determination of the biological value of casein for man based on results of balance investigations of nitrogen metabolism in 30 normal males are reported. These were kept on standard semisynthetic diets differing in the daily content of caseine (from 0 up to 15.1 g of nitrogen per day) that served as a sole source of protein in the ration. The levels of endogenous losses of nitrogen with the urine, feces and summarily in any other ways were determined. Principal features of methodological procedures employed in determining the biological value of proteins recommended for nutrition of man are substantiated. The need for multilevel (not less than on 3-4 levels) investigation, below the one necessary for maintaining nitrous equilibrium, is stressed.