Fluorescently labeled and conjugated (strept)avidins are widely used for imaging biotinylated molecules in immunological assays and histochemistry. We showed that besides biotin, these proteins bind glycans, including fragments of mammalian glycoproteins and glycolipids, in particular, ABO blood group antigens, oligolactosamines, and 6--sulfated oligosaccharides. This interaction is inhibited in a dose-dependent manner by micromolar concentrations of polymeric, but not monomeric, glycan conjugates (i.e., requires polyvalence). Taking into account the cluster organization of cell glycans (glycoproteins and glycolipids), the ability of (strept)avidins to bind glycans might be a source of errors in the analysis of carbohydrate-containing samples, which can be prevented by avoiding a large excess of (strept)avidin in analytical systems.