From the preliminary observations described in this report, the following conclusions may be drawn. When attempting to produce diagnostic antiserums specific for S mutans, S mitior, or S sanguis, care must be taken to eliminate cross-reactions as a result of antibodies against extracellular glucans or dextrans. Other antigenic components, associated with the cell wall, may be common to different species. One soluble protein or polypeptide antigen normally present in S sanguis and S mitior may also be demonstrated in certain strains of S mutans. Antiserums prepared against S sanguis, S mitior, or S mutans should be carefully checked for interspecies cross-reactions before being used for identification purposes.