Variation in apparent ileal digestibility of amino acids within the same diet is partly related to the amino acid composition of feed protein and endogenous protein. Mathematical model which estimate the apparent ileal amino acid digestibility coefficients, based on apparent ileal digestibility coefficients of the diet protein and the amino acid composition of the diet protein as factors, has been described in literature. In the present study, apparent ileal amino acid digestibilities of pigs fed eighteen different diets with a wide range of different protein sources were determined and the mathematical model from the literature was validated. Results of the validation showed that the mathematical model explained 60-94% of the variation in apparent ileal amino acid digestibility of sixteen diets. However, variations for diets with Phaseolus beans and meat and bone meal as protein sources were explained by only 45 and 27%, respectively. Standard error of prediction (SEP) was about 2-3 times higher as compared to the standard error of the mean (SEM) of the determinations between animals. For most of the feedstuffs, the factors of the model explain the main part of the variation in apparent amino acid digestibility ( R² ≥ 0.60). If the explained part of the variation in apparent amino acid digestibility is small, R² < 0.50, other factors which are not included in the model have major effects on apparent digestiblity of the amino acids.