The apparent and true ileal digestibility of protein and amino acids of five yellow lupin cultivars grown in Poland was estimated on growing castrated male pigs. The nitrogen retention was determined on young boars fed a barley-yellow lupin diet supplemented with crystalline lysine, methionine and tryptophan. The apparent ileal digestibility of the seed protein was from 81.9 to 84.9%. There were no significant differences in protein or in amino acid apparent digestibility among the yellow lupin cultivars. The true ileal digestibility of crude protein was from 87.5 to 90.5%. Nitrogen retention was significantly lower (P<0.05) in pigs fed the diet supplemented with amino acids according to pig requirements for total amino acids (methionine constituted 36% of the sulphur containing amino acids) than in those fed the same diet but with higher methionine content (50% of sulphur containing amino acids) or the diet supplemented according to the requirements for ileal digestible amino acids.