The relative affinity of various oral streptococci for hydroxyapatite (HA) differed widely. Streptococcus mutans affinity for dextran-coated HA was the highest of all the streptococci to any HA coating. S salivarius had the lowest affinity, and S sanguis affinity was higher than S miteor for saliva-coated HA. Competition for binding sites on saliva-coated, dextran-coated, and saliva-dextran-coated HA among these streptococci was not observed in any pair of these bacteria on any HA coating except for that of S miteor and S sanguis to saliva-coated HA. However, competition between this pair was not for the same binding site but for closely associated sites.