The inhibitory potencies of a number of mannosides, di- and trivalent mannosides, a set of mannose-terminating dendrimers, and five types of mannose-bearing neoglycoproteins were determined by using a binding assay that measures the binding of I-125-labeled, highly mannosylated neoglycoprotein to a type 1 fimbriated Escherichia coli (K72) strain In suspension. The IC50 values (the concentration of inhibitor that causes 50 % reduction-in the bound I-125-ligand to E. coli) obtained by this method were much lower than the equivalent values obtain hemagglutination or in assays that involve microplate immobilization. Two important factors that strongly influence the affinity to E. coli adhesin are: 1) the presence of an alpha-oriented aglycon that has a long aliphatic chain or an aromatic group immediately next to the glycosyl oxygen, and 2) the presence of multiple mannosyl residues that can span a distance of 20 nm or longer on a relatively structure. The two best inhibitors, which ore a highly mannosylated neoglycoprotein with the longest linking arm between a mannose and protein amino group and the largest mannosylated dendrimer (fourth generation), exhibited sub-nM IC50 values.