In quantitative experiments using ELISA, binding of Tamm-Horsfall protein (THP) to uropathogenic Escherichia coli was studied with monoclonal antibody to THP. Adherence to E. coli bearing type 1 fimbriae was proportional to THP concentration and size of the bacterial inoculum. Type 1 fimbriae-bearing E. coli bound 50 times more THP than did non-type 1-fimbriated or P-fimbriated strains. Concanavalin A and wheat germ agglutinin bound THP in a dose-dependent fashion, whereas pokeweed mitogen and Vicia villosa B4 isolectin did not. Addition of mannose and N-acetylglucosamine reduced adherence of THP to concanavalin A and wheat germ agglutinin by 50%-80%. Sugar inhibition studies suggested that the fimbrial receptor site for THP has lectin-like properties and that THP binds to fimbriae via its mannose side chains. This quantitative assay is useful for studying the interaction between THP, uroepithelial cells, and bacteria in vitro.