The effect of the protein content in the diet on the tissue composition of 90 male Polish Lowland lambs fed diets containing from 13 to 18% crude protein in DM during the fattening period and on 44 carcasses of male Polish Merino lambs fed diets containing from 11 to 19% protein in DM was studied. The energy value of the diets was similar. The lambs were fattened from weaning at the age of 70–90 days to reaching a slaughter weight of 35–40 kg (Polish Merino) or 35–45 kg (Polish Lowland). The Lowland lambs that received less protein in diets had somewhat lower lean content and higher fat content in the carcass than the lambs that were fed diets with a higher protein level. No significant differences were found in the carcass composition of Merino lambs. The tissue composition of the carcasses depended more on the slaughter weight of the lambs and the related carcass weight than on the protein level in the diet.