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Strains of the bacterium Escherichia coli that cause infections of the human urinary tract produce so-called Pap-pili, which are hair-like appendages consisting of about 103helically arranged subunits of the protein PapA. These pili mediate binding to digalactoside-containing glvcolipids present on the epithelial cells which line the urinary tract1,2. Recently, it has been suggested that three proteins, PapE, PapF and PapG, are responsible for this binding3,4. In the absence of PapA, non-piliated bacteria are formed which nonetheless exhibit binding, showing that the bulk of the pilus is not essential for binding3,5. Although pili can form without PapF and PapG, such pili are unable to bind to the digalactoside3. The protein PapG mediates binding specificity in trans-complementation experiments, so this protein is the digalactoside-specific adhesin6. Using immune-electron microscopy we have found that Pap-pili are heteropolymers composed of the major pilin, PapA, the minor pilins, PapE and PapF, and the adhesin, PapG. The last three proteins are located at the tip of the pilus.