Alpha-mannosyl glycoclusters and glycodendrimers were tested as multivalent inhibitors of the type 1 (mannose-specific) fimbriae of a recombinant E. coli HB101 strain. Inhibition of haemagglutination of guinea pig erythrocytes was determined on microtiter plates. The effect of multivalency is pronounced for up to three mannosyl residues in the molecule, whereas larger derivatives do not have an appreciable effect on binding to the fimbrial carbohydrate binding domain. The best glycoclusters tested reach the binding potency of the known potent inhibitor pNPMan (3). The results support the idea of a monovalent recognition site at the adhesive protein FimH, which might best accommodate molecules with the size of a trisaccharide or those which expose up to three alpha-mannosyl residues, such as the glycocluster 8. The results obtained with the thiourea-bridged alpha-mannosyl clusters, possessing defined sugar valencies, facilitate the development of high affinity inhibitors of the fimbrial lectin on type 1 fimbriae.