1. alpha 1-Proteinase inhibitor (alpha 1-antitrypsin) is excreted in a deglycosylated form (M(r) 38,000) in the faeces of healthy subjects and in patients with quiescent Crohn's disease. By contrast, in most patients with active Crohn's disease, alpha 1-proteinase inhibitor is excreted in a glycosylated form (M(r) 51,000). 2. Faecal extracts containing deglycosylated alpha 1-proteinase inhibitor are able to deglycosylate alpha 1-proteinase inhibitor by an exoglycosidic process. Conversely, we demonstrate that in faecal extracts from patients excreting glycosylated alpha 1-proteinase inhibitor, glycosidase activities, such as N-acetyl-beta-glucosaminidase (EC 3.2.1.30), alpha-mannosidase (EC 3.2.1.24) and particularly beta-galactosidase (EC 3.2.1.23), are strongly decreased. 3. Degradation of glycosidases by proteases could not explain the decreased glycosidase activity in these faecal extracts. 4. Our data suggest that a modification of the bacterial colonic flora (or of its metabolic activity) occurs in most patients with active Crohn's disease and could be responsible for an impaired colonic salvage of carbohydrates.