Aims: Adhesion of a micro-organism to a cell surface is often considered to be the first step in pathogenesis. Inhibiting this process may have therapeutic effects in vivo. This study investigates the inhibitory effects of various bovine whey products on the association of Salm. Typhimurium, E. coli O157:H7 and C. malonaticus (formerly Enterobacter sakazakii) to the human CaCo-2 cell line. Invasion of CaCo-2 cells by Salm. Typhimurium and C. malonaticus was also examined. Methods and Results: Infection assays were performed by incubating pathogenic acteria with CaCo-2 cells in the presence of untreated (UT) or enzyme-modified (EM) whey products. Associated micro-organisms were directly quantified by plate counts. Invasion of CaCo-2 cells by Salm. Typhimurium and C. malonaticus in the presence/absence of test materials was also quantified using gentamicin protection assays. At a concentration of 40 mg ml-1, some UT whey products reduced association and invasion, but this effect was enhanced following hydrolysis with porcine pancreatic lipase. Conclusions: Both UT and EM sweet whey protein concentrates (WPCs) were found to be particularly effective inhibitors of association and invasion. All EM whey products significantly (P < 0 center dot 05) inhibited invasion of C. malonaticus into epithelial cells, causing a 2-log reduction in the quantity of these micro-organisms internalized. Significance and Impact of the Study: The present study suggests that whey products can inhibit association to and invasion of CaCo-2 cells by selected micro-organisms and may be useful in the treatment and/or prevention of foodborne infections.