A spontaneous non‐adherent mutant (Adh−) of Streptococcus sanguis, strain G9B has been isolated from a chemostat culture employing a chemically defined medium (FMC) as a nutritional source. The Adh− mutant showed five‐fold less adhesion to saliva‐coated hydroxyapatite (SHA) compared with parent G9B. No difference in adhesion to uncoated hydroxyapatite (HA) was found between G9B and Adh−. The mutant had the same colonial, cellular morphology, and biochemical properties as the parent. A comparison of the antigens found in mutanolysin (M‐1, N‐acetylmuramidase) extracts of G9B and Adh− using crossed rocket immunoelectrophoresis (CIE), indicated that a surface antigen, designated A, reacted strongly in G9B but only weakly in Adh−. The adhesion of G9B but not Adh− to SHA was directly correlated with dilution rates in a chemostat. Adhesion to HA was not affected by the dilution rate. There was also a direct correlation between the concentration of the A antigen with adhesion of G9B to SHA. The concentration of A in Adh− was low at all dilution rates. Antibody to G9B specifically inhibited adhesion of G9B to SHA but antibody to Adh− had no effect on adhesion of either strain to SHA. Neither antibody affected adhesion to HA. A partially purified A antigen absorbed adhesion inhibitory activity of G9B IgG. Trypsin did not digest the A antigen but resulted in the extraction of the antigen from the surface causing the loss of the cells ability to adhere to SHA. Galactose oxidase treated G9B cells also showed a reduced adhesion to SHA but glucose oxidase had no effect. Periodate oxidation of G9B resulted in a loss of adhesion to SHA. The A antigen was found in purified cell walls and was labelled with 125I, indicating a surface location. The A antigen also reacted with affinity purified antibody against the G9B adhesin antigen. Immunoblots of the A antigen following SDS‐PAGE indicated it contained 80, 62, and 52 kDa polypeptides. These results suggest that the A antigen is a glycoprotein which is the native form of the adhesin responsible for the attachment of G9B to salivary pellicle.