The chemical and amino acid composition of the empty bodies of 4 lambs slaughtered before fattening and of 12 lambs from 3 gropus (4 in each group) fattened for 113 day with a diet containing 17 (H), 14 (M) or 11% (L) crude protein were determined. The amount of nitrogen and individual amino acids apparently absorbed in the small intestine during fattening was calculated on the basis of data on their passage to the duodenum and digestibility in the small intestine. No differences were found in empty body composition depending on the protein content of the diet. The mean daily deposition of protein in the empty body was: 29.7,31.4 and 27.3 g in groups H, M and L, respectively. The utilization of protein for deposition into the empty body weight rose (P≤0.01) from about 18 to 27% when the protein content was decreased from 17 to 11%. The utilization of metabolic energy available for growth (Kf) was best in group M and amounted to 0.41 in comparison with 0.38 and 0.33 in groups H and L, respectively. The lowering of the protein level in the diets improved the efficiency of utilization of amino acids absorbed in the smali intestine for their deposition in the empty body weight. Essential amino acids and semi-essential amino acids were best utilized by lambs from group M. From all of the amino acids, cystine and glycine were utilized best, with cystine being utilized better in groups H and M than in L. Reducing the protein content in group L to about 11% resulted in decreasing the percentage of essential amino acids in protein deposited in the body, especially in comparison with the group fed diet M. It seems that the most effective utilization of protein takes place at a 14% crude protein content in the diet.