Some strains of Streptococcus mutans were found to recognize and bind collagen type I. Binding of 125I-labeled collagen type I was specific in that collagen types I and II, but not unrelated proteins, were able to inhibit binding of the labeled ligand to bacteria. Collagen binding to S. mutans was partially reversible and involved a limited number of bacterial binding sites per cell. S. mutans UA 140 cells bound collagen type I with high affinity (Kd = 8 x 10(-8) M). The number of binding sites per cell was 4 x 10(4). Collagen-binding strains of S. mutans were found to adhere to collagen-coated surfaces as well as to pulverized root tissue. S. mutans strains that did not bind the soluble ligand were unable to adhere to these substrata. Adherence to collagen-coated surfaces could be inhibited with collagen or clostridial collagenase-derived collagen peptides. Adherence of S. mutans to dentin was enhanced by collagen types I and II but inhibited by collagen peptides. S. mutans UA 140 bound significantly less 125I-collagen type I following treatment with peptidoglycan-degrading enzymes. These enzymes released a collagen-binding protein (collagen receptor) with a relative molecular size of 16 kDa. The results of this study suggest that collagen mediates adhesion of S. mutans to dentin. This interaction may target collagen-binding strains of S. mutans to dentin in the oral cavity and may play a role in the pathogenesis of root surface caries.