Rapid agglutination of Actinomyces viscosus and Actinomyces naeslundii cells by D-mannose solutions was observed during studies of their attachment to mammalian cells in vitro. The specificity of the agglutination reaction was studied by slide agglutination tests and by measuring the rate of decrease in optical density of bacterial phosphate buffer suspensions caused by the setting of bacterial aggregates. Actinomyces cells were agglutinated by protein-containing mannose solutions of several chemical suppliers. Solutions of sugars other than D-mannose and solutions of mannitol and mannan all failed to agglutinate A. viscsus and A. naeslundii. "Mannose-enhanced" agglutination was impaired by boiling or autoclaving the mannose but was not affected by heating the bacteria, the presence of chloramphenicol, running the assay in the cold, or incorporating any of several commercially purchased sugars in the reaction mixture. During these hapten inhibition experiments, only 6-deoxy-L-talcose-containing extracts of an A. viscosus strain retarded the rate of mannose-enhanced agglutination. Protein-containing fractions of D-mannose mother liquors also agglutinated cells of A. viscosus and A. naeslundii. Other species of oral gram-positive rods were not agglutinated by mannose solutions. Together the data indicate that plant seed-derived D-mannose contains a protein-associated agglutinin for A. viscosus and A. naeslundii which may function via a "lectin-like" selective affinity for the unique cell wall sugar 6-deoxy-L-talose.