The adhesion to whole and fractionated porcine gastric mucus of both Lactobacillus fermentum 104-S cells and a saccharide extracted from this strain was investigated. It has been shown previously that this saccharide had affinity for nonsecreting gastric epithelium. The mucus component(s) with affinity the bacterial cells was partly characterized by gel filtration and treatment with protease or metaperiodate. L. fermentum 104-S extracts containing the saccharide were radioactively labeled, fractionated by gel filtration, and tested for affinity for the gastric mucus component showing receptor activity for the whole cells of strain 104-S. The mucus material with affinity for the bacterial cells had a relative molecular weight of 30-70 K. From the results of treatment with protease or metaperiodate, it is proposed that the mucus components(s) that adhered to the whole bacterial cells contained glycoprotein groups. The radioactively labeled saccharide extracted from L. fermentum 104-S cells did not bind to the mucus fraction that had affinity for the whole cells. Conclusively, we suggest that the mechanism by which cells of L. fermentum 104-S adhere to the gastric mucus is different from the mechanism mediating the adhesion of this strain to the nonsecreting gastric epithelium. Cells of L. fermentum 104-S adhere to a glycoproteinaceous mucus component with a relative molecular weight of 30-70 K.