Streptococcus mutans is a major pathogen of dental caries and also considered to be involved in infective endocarditis (IE). S. mutans strains are serologically classified into four serotypes, c, e, f, and k. Serotype c is the major type found in oral isolates from healthy subjects with a distribution frequency of approximately 70% to 80%, followed by serotype e (approximately 20%), while that for serotypes f and k is lower than 5%. On the other hand, serotype k strains have been identified with high frequency in S. mutans-positive extirpated heart valves specimens from IE patients.An approximately 120-kDa Cnm protein related to the collagen-binding activity of S. mutans was characterized in 2004. In our previous study, we identified several clinical strains of S. mutans, most which were serotype k, that did not possess the cnm gene but exhibited collagen-binding activity. Thus, we speculated that other unknown proteins with collagen-binding properties may exist in these serotype k strains, after which another collagen-binding protein of S. mutans was identified and named Cbm. In related experiments, the binding activity to type I collagen of strains positive for cbm encoding Cbm was significantly greater than that of cnm-positive strains. In addition, S. mutans strains possessing collagen-binding proteins and lacking expression of the cell surface 190-kDa protein antigen (PA) had high properties of adhesion to and invasion of human umbilical vein endothelial cells, which may be associated with virulence for IE.