Four out of 522 streptococcal isolates from the peripheral blood of patients with bacteremia exhibited typical properties of Streptococcus mutans in terms of sucrose-dependent adhesion, expression of glucosyltransferases, fermentation profiles of sugars, the presence of surface protein antigen, and DNA-DNA hybridization. Two strains were determined as serotype f and e by immunodiffusion, whereas the other two isolates did not react with the specific antiserum to S. mutans serotype c. e. or f of the eight different serotypes of mutans streptococci. The latter two untypable isolates, however, expressed a new antigenic determinant that was different from serotype c/e/f specificity as revealed by immunodiffusion. Analysis of the cell wall polysaccharides revealed very low contents of glucose in the untypable isolates. Furthermore, Southern blot analysis demonstrated that the untypable strains lacked at least one gene corresponding to a glucose-adding enzyme. These results indicate that the serologically untypable nature is due to the loss of glucosidic residue from the serotype-specific polysaccharide antigens of S. mutans.