The role of leucine aminopeptidase (LAP) from pig kidney cytosol and X-prolyldipeptidyl aminopeptidase (Pep X) from Lactococcus lactis subsp. cremoris AM2 in the hydrolysis and debittering of a tryptic digest of ␤ ␤ ␤ ␤ ␤-casein was studied. Hydrolysis was monitored by quantifying the release of primary amino groups and bitterness by use of a trained sensory panel. Sequential incubation of the bitter tryptic hydrolysate with LAP, Pep X and LAP resulted in higher levels of hydrolysis and significantly (P < 0.001) lower levels of bitterness than incubation with LAP alone. The results demonstrate the central role proline-specific aminopeptidases can play in the hydrolysis and debittering of food protein hydrolysates.