Strains of Streptococcus cricetus and Streptococcus rattus exhibited striking differences in their ability to bind to different types of collagen. For example, S. cricetus AHT bound in highest numbers to hydroxyapatite (HA) treated with human type V collagen, while rat type I collagen was ineffective. In contrast, human type V collagen was least effective in promoting attachment of S. rattus LB-1, while treatment with rat or human type I collagen was effective. Adsorption of both species to human type I collagen-treated HA showed a high correlation with a Langmuir model. Estimates of adsorption parameters indicated there were greater numbers of binding sites with higher affinity for S. rattus LB-1 than for S. cricetus AHT. Treatment of HA with either the alpha 1 (1) or alpha 2 (1) polypeptide chains of collagen was effective in promoting adhesion of S. rattus LB-1 cells. In contrast, the alpha 2 (1) chain was more effective than the alpha 1 (1), chain for S. cricetus AHT. These observations indicate that S. cricetus AHT and S. rattus LB-1 cells bind to different segments of collagen molecules. Adhesion of both species was also promoted by collagen-rich fractions of human dentin.