In Swiss-type cheese such as Emmental, proteolysis is one of the major phenomena occurring during ripening. Among the proteolytic agents involved in cheese ripening, the free enzymes originally present in milk and those arising from bacterial autolysis can act directly on the casein network. In order to understand the contribution of the bacterial enzymes and especially those arising from the thermophilic starters, the juice of an Emmental cheese entering the warm room was extracted by pressure, then sterilized by filtration and incubated at 24°C for 20 d under anaerobiosis. At different times, the peptides and free amino acids were determined in the sterile cheese juice. In parallel, in order to gather information about the nature of the enzymes present, the sterile juice was also incubated with β-naphthylamide derivatives as substrates. We have demonstrated a continuous increase in free NH2 groups and in free amino acids throughout the 20 d incubation time. The main peptidase activity was due to aminopeptidase(s) and X-prolyldipeptidyl aminopeptidase(s) whose activities were recovered after non-denaturing polyacrylamide gel electrophoresis. Most of the enzymes found in the juice would have their origin in thermophilic starters. As they are generally intracellularly located, their release could be explained by the autolysis of these starters. Finally, the main free amino acids released in the juice (Pro, Glu, Ala, Val, Leu and Lys) corresponded to those previously found in Emmental cheese, suggesting that the enzymes detected in this study participate significantly in peptide degradation during ripening.