AbstractTotal nitrogen, amino acid nitrogen, and true protein were determined in seven common feedstuffs (protein sources): skim milk powder, barley, grass meal, soybean meal, fish meal, peas, and meat and bone meal. The conversion factors from total nitrogen and amino acid nitrogen, respectively, to true protein were calculated in the investigated protein sources. A considerable variation in the conversion factors was found. Furthermore, in most cases the conversion factor from total nitrogen to true protein were much lower than the commonly used factor of 6.25. The results were explained by the content of other nitrogen containing compounds and a considerable variation in the amino acid composition of true protein in the different protein sources. Occurring problems by using the factor 6.25 exclusively are briefly discussed in the light of the present findings.