THE colonisation of bacteria on human mucosal tissues is now recognised as an important step in the infectious process1,2. To colonise mucosal surfaces, the organisms must first bind to the epithelial cells of these tissues, or they are discarded by the host's physico-anatomical defence mechanism (sneezing, coughing, urine flush and so on). The attachment of bacteria to epithelial cells can be demonstrated in vitro3,4 and is probably mediated by surface components of both types of cell5. It was found4,5 that lipoteichoic acid on the surface of group A streptococci is responsible for the attachment of these organisms to oral mucosal cells. For Gram negative organisms, pili mediate their binding to epithelial cells6-9. In a study with Proteus mirabilis it was found10 that the presence of pili is important for the organisms to adhere to both oral and bladder mucosal cells as well as to initiate retrograde infection in experimental animals, probably by enabling the bacteria to adhere and to colonise pelvic epithelial cells11. Nothing is known, however, about the putative receptor sites required for bacterial attachment on the mucosal cell membrane. We present here data indicating that attachment of Escherichia coli to epithelial cells is mediated by mannose (or mannose-like) receptors present on the surface of the latter.