Twelve crossbred barrows (BW 47 +/- 2.81 kg) were fitted with a T-cannula in the terminal ileum and fed one of ten diets containing corn gluten meal, dried distillers grain or dried distillers grain with solubles as the sole protein source, according to a 6 x 6 Latin Square design. Lysine, tryptophan and methionine were, the first, second and third limiting amino acids in these by-products. The ileal apparent digestibility of methionine, tyrosine, leucine and phenylalanine in corn gluten meal was high. The ileal apparent digestibility of tryptophan, tyrosine and leucine differed significantly in corn gluten meal. The ileal apparent digestibility of amino acids was higher in dried distillers grain with solubles than in dried distillers grain. To predict the ileal apparent digestibility of tryptophan, methionine and threonine linear regression equations were derived. The results suggested that processing technique can influence the ileal apparent digestibility ofamino acids in corn by-products. It was concluded that the direct technique cannot accurately determine the ileal apparent digestibility of tryptophan in dried distillers grain with solubles and dried distillers grain, due to its low content. The content of an amino acid in a feedstuff used as the sole source of protein must meet the minimum requirement of pigs when the ileal apparent digestibility of constituent amino acids is determined using the direct technique.