The combining sites of type 1 fimbrial lectins of various species of enterobacteria were studied by measuring the inhibitory activity of linear and branched oligosaccharides and several glycosides of D-mannose on the agglutination of yeast cells by the organisms. The results showed that all five strains of Escherichia coli tested possessed an elongated combining site best fitting a trisaccharide and including a hydrophobic region. Similar results were obtained with Klebsiella pneumoniae. Within the Salmonella genus, the combining sites of the six species tested were similar, but all differed significantly from those of the E. coli strains. The combining sites of Enterobacter cloacae and Enterobacter agglomerans were different from each other and from those of Salmonella sp. and E. coli. The results suggest that although classified under the general term 'mannose-specific', bacterial lectins in the form of type 1 fimbriae on different genera exhibit differences in sugar specificities.