Campylobacter upsaliensis is a recently recognized human enteric pathogen associated with enteritis, colitis, bacteremia, and sepsis. Very little is known about the mechanisms of pathogenesis of this organism. The goals of this study were to determine whether C. upsaliensis binds to epithelial cells and whether there are specific lipid molecules that might serve as cell membrane receptors. In addition, we also explored C. upsaliensis binding to purified human small-intestinal mucin, since the mucus gel overlying the epithelium provides an initial contact surface for the bacteria and must be penetrated for the organisms to reach their cell receptors. Binding of C. upsaliensis to model epithelial cells was shown by microscopy adhesion assays, and binding to lipids was detected by thin-layer chromatography-overlay assays. Bacteria bound to phosphatidylethanolamine (PE), gangliotetraosylceramide (Gg4), and, more weakly, to phosphatidylserine (PS). There was no binding to ceramide, cholesterol, phosphatidylcholine, and globosides. Using receptor-based microtiter well immunoassays, we observed binding to be equal, specific, and saturable for PE and Gg 4 but low and nonspecific for PS. At least five bacterial surface proteins (50 to 90 kDa) capable of PE binding were identified by a lipid-silica affinity column technique. In slot blot overlay assays, biotin-labeled C. upsaliensis also bound in a concentration-dependent fashion to purified human small-intestinal mucin, implying that these microorganisms also express an adhesin(s) recognizing a specific mucin epitope(s). We speculate that binding to mucins may influence access of the bacteria to cell membrane receptors and thereby influence host resistance to infection.