The mechanism by which Lactobacillus fermentum strain 104-S adheres to porcine squamous epithelium was investigated by studying the adsorption to epithelial cells, and control surfaces, of radioactively labelled material released from the bacterial cells by water extraction. The released material was fractionated by gel filtration and the adsorption of pronase-sensitive and -resistant material in the various fractions to porcine gastric tissue and the control surfaces of polystyrene and immobilized bovine serum albumin (BSA) was determined. The fraction with affinity for the epithelium was characterized by enzymic degradation, periodate oxidation, lipid extraction, and protein and carbohydrate analyses. The adsorption pattern of radioactively labelled crude released material mimicked the adhesion of whole labelled cells to polystyrene and to gastric squamous tissue pieces. On fractionation, the pattern of adsorption to polystyrene and BSA was different from that obtained for the tissue pieces. Considerably less labelled pronase-stable material bound to surfaces of polystyrene and BSA, as compared with the tissue, suggesting that the pronase-resistant component has a tissue-specific affinity. After pronase treatment of the fraction of M(r) about 20,000 (20 K) containing labelled components with affinity for the epithelium, only saccharides were detected. Radioactivity was lost after hydrolysis with HCl, and therefore this pronase-resistant labelled component must be a saccharide. It is concluded that protein moieties in the extract have an affinity for several surfaces, including polystyrene, and that saccharide moieties have a specific affinity for the gastric squamous epithelium.