Treatment of hydroxyapatite (HA) with human type I or type III collagen strongly promoted adhesion of Actinomyces viscosus LY7 cells. Treatment with human type V collagen was somewhat less effective while treatment with human type IV or rat type I collagen was significantly less effective. Electron microscopic observations revealed that A. viscosus cells also attached to fibrils prepared from human type I collagen. The alpha 1 (1) polypeptide chain derived from type I collagen was ineffective in promoting binding and the alpha 2 (1) polypeptide chain exhibited moderate activity. Heat- or urea-denatured type I collagens were also ineffective in promoting binding. Mutants of A. viscosus that possess type 1 fimbriae, but not type 2 fimbriae or no fimbriae, also bound to collagen-treated HA; this suggests that the adhesin responsible was associated with type 1 fimbriae. Strains of Actinomyces israelii and Actinomyces odontolyticus also exhibited strong binding to collagen-treated HA, while Actinomyces naeslundii ATCC 12104 did not. The avidity of Actinomyces species for collagen would seem to be at least partially responsible for the high proportions of these organisms found on cemental and root tooth surfaces.