A protein, designated antigen III, was extracted from cells and culture supernatant of Streptococcus mutans serotype c, and purified by ammonium sulphate precipitation followed by chromatography on anion-exchanged and gel filtration columns. Monospecific antisera were raised by injecting purified antigen III in rabbits. Antigen III prepared from cells and supernatants appeared identical by reaction with antisera and were of similar chemical composition and physico-chemical properties. Antigen III was resistant to most proteolytic enzymes, but pepsin digestion decreased its molecular weight from 44,000 to 24,000 without destroying antigenic activity. Immunofluorescence showed that antigen III was located at or near the cell surface of S. mutans serotypes b, c, e and f.