Protein digestibility, a key indicator of dietary protein quality, can be estimated in vitro. The objectivewasto determinethe contribution of endogenous N, released by the enzyme autolysis, to in vitro protein digestibilityusing 15N-labelled proteins. Methods:15N-labelled gluten and caseins (4, 8 and 16% of the meal) were digested using the INFOGEST static in vitromodel.The endogenous N was measured after centrifugation and 10-kDa ultrafiltration, allowing the true N digestibilitydetermination. The proteolysis degree, based on the primary amine content, was determined. Results: Alarge proportion (93±11%) of the endogenous N < 10 kDa within digesta was present before digestion, with somecontribution, although limited(20-30%),of the enzyme autolysis. For both proteins, true N digestibility was not significantly different between 30and120 min of intestinal digestion,but significantly higher for the lowest protein content (4%). The proteolysis degree was more correlated to theapparentdigestibility, i.e.withno endogenous N correction, than to the true digestibility, showing that further endogenous correction is needed for more accuracy.Conclusion: In vitro protein digestibility, as well as proteolysis degree, needs to be corrected for the endogenous N contribution, although to a different extent.