A large number of linear and branched oligosaccharides and several glycosides of D-mannose were tested for their inhibitory activity on the agglutination of yeast cells or guinea pig erythrocytes by three D-mannose-specific enteric bacteria possessing type 1 fimbriae. With Escherichia coli 346, the best inhibitors found are the alpha glycosides of the branched oligosaccharides alpha-D-Manp-(1 leads to 3)-[alpha-D-Manp-(1 leads to 6)]-alpha-D-Manp-(1 leads to 6)-alpha-D-Manp-(1 leads to 3)-D-Manp and alpha-D-Manp-(1 leads to 3)-[alpha-D-Manp-(1 leads to 6)]-alpha-D-Manp- (1 leads to 6)-[alpha-D-Manp-(1 leads to 2)-alpha-D-Manp-(1 leads to 3) ]-D-Manp and the trisaccharide alpha-D-Manp-(1 leads to 3)-beta-D-Manp-(1 leads to 4)-D-GlcNAc, all of which are 21-30 times more inhibitory than methyl alpha-D-mannopyranoside. The aromatic glycoside p-nitrophenyl alpha-D-mannopyranoside was also a strong inhibitor (30 times more inhibitory than methyl alpha-D-mannopyranoside), whereas the corresponding beta-D-glycoside was only a weak inhibitor (approximately as methyl alpha-D-mannopyranoside). A nearly identical pattern of inhibitory activity was observed with the fimbriae. This suggests that the combining site of the E. coli fimbrial lectin is in the form of an extended pocket on the surface of the lectin corresponding to the size of a trisaccharide and fitting best the structure alpha-D-Manp-(1 leads to 3)-beta-D-Manp-(1 leads to 4)-D-GlcNAc. Since p-nitrophenyl alpha-D-mannopyranoside is a strong inhibitor, the existence of a hydrophobic region in the combining site or close to it was assumed. The combining site of the Klebsiella pneumoniae fimbrial lectin is probably similar to that of E. coli, but that of the Salmonella typhimurium fimbrial lectin differs considerably. It appears that the combining sites of the three bacterial lectins tested exhibit preference for structures found in N-glycosylic oligomannoside units of mammalian cell surface glycoproteins.