The effect of treatment of Lactobacillus fermentum with several protein- and carbohydrate-modifying reagents on the bacterium's ability to flocculate Saccharomyces cerevisiae was investigated. The proteinaceous nature of the cell-surface components of L. fermentum which are responsible for flee formation was confirmed by inactivation of flee formation following photo-irradiation, with Methylene Blue or Rose Bengal as sensitizer, or acylation with acetic anhydride, maleic anhydride or acetylimidazole, and by the reaction of the components with nitrous acid, I-2 and performic acid. The phenolic hydroxyl group of tyrosine and the indole group of tryptophan appear essential for flocculation. Proteinaceous components of the yeast cell surface and carbohydrate components on the bacterial cell surface were not required for flocculation but carbohydrate residues on the yeast surface were essential.115508511