A bacterial pathogen that is important in both urinary tract and intestinal infections is Escherichia coli which expresses Dr or related adhesins. In this report, we present a model for testing cell-cell interaction, using both molecularly characterized laboratory cells that express recombinant molecules of human decay-accelerating factor (DAF), and recombinant bacterial Dr colonization factors. Dr adhesin ligand was identified as DAF (CD55), a membrane protein that protects autologous tissues from damage due to the complement system. Structure-function studies mapped the adhesin-binding site on the DAF molecule. A single-point substitution in the third short consensus repeat domain, Ser165 to Leu, corresponding to the Dra to Drb allelic polymorphism, caused complete abolition of adhesin binding to DAF.