The main part of strains of Streptococcus mutans isolated from the present Danish material of blood from patients with subacute endocarditis and from human teeth belonged to two of five serotypes established by Bratthall, viz. type c and type e. Two new types were established: type f and type g. Strain SL-1 seems to constitute a distinct type. Strains of serotypes a and b have not been isolated in Denmark, and strains of serotypes d, g and SL have been isolated from teeth only. The registered differences in biochemical behaviour warrant a proposal of a subdivision into three biotypes.