Five strains of human fecal bacteria, of the Ruminococcus and Bifidobacterium genera, produce extracellular α- and β-glycosidases that degrade intestinal mucin oligosaccha-rides and glycosphingolipids of the lacto-series type 1 chain. We have tested the activities and substrate specificities of these enzymes using para-nitrophenyl glycosides and glycosphingolipids of different core chains (lacto, neolacto, globo, isoglobo, galabio, and ganglio), carrying different blood group determinants (A, H, X, Y, Forssman, and para-Forssman), and with different degrees of sialylation (mono- to tetra-sialo). Lactotetraosyl-ceramide and neolactotetraosylceramide were the only core glycosphingolipids degraded by enzymes from these strains, resulting in lactosylceramide and glucosylceramide as the major end products. R. gnavus strain VI-268 did not degrade lactotetraosylceramide but only neolactotetraosylceramide yielding lactotriaosylceramide and lactosylceramide as the major end products. All strains but R. gnavus VI-268 also produced lactosylceramide from a bi-antennary 10-sugar glycosphingolipid with two blood group H determinants based on a lactotetraosylceramide core. Apart from strain specific blood group A-degrading (R. torques strain VIII-239 and IX-70, R. gnavus strain VI-268 and B. infantis VIII-240) and Forssman-degrading (R. torques VIII-239 and IX-70) activities, all strains also degraded the II-5, X-5, and Y-6 glycosphingolipids. All strains released N-acetylneur-aminic acid from the gangliosides sialosyl-neolactotetraosylceramide, GD3, GD1a, GD1b, GT1b, and GQ1b, corresponding to 2, 3-α- and 2, 8-α-N-acetylneuraminidase activities. The R. torques strains VIII-239 and IX-70 also partially desialylated GM1 to lactotetraosyl-ceramide. The para-nitrophenyl glycoside degradations were often incompatible with the data from the glycosphingolipids degradations. We conclude that the high degree of substrate specificities of these bacterial glycosidases is primarily directed towards the main oligosaccharides structures produced in the human gastrointestinal epithelium, i.e. lactoseries type 1 and type 2 chains. Their degradation may, through the production of nutrients, growth factors and receptors be important for establishment and maintainance of the intestinal microbiota.