Experiments utilizing radio labelled oral streptococci in an in-vitro assay system have demonstrated the selective ability of certain species of these bacteria preferentially to adhere to saliva-coated and dextran-coated hydroxyapatite (HA). Pretreatment of Streptococcus sanguis and Streptococcus mutans with the proteolyic enzymes trypsin and pronase greatly reduced their ability to adhere to saliva or dextran-coated HA respectively. Pretreatment of Strep. sanguis with lipase increased its adsorption to saliva-coated HA and stearic acid-blocked adsorption. Cell walls isolated from Strep. sanguis proved effective in blocking Strep. sanguis adsorption to salivacoated apatite. Cells of Strep. mutans after treatment with dextranase or low molecular weight dextrans did not adhere well to dextran-coated HA.