Serotype h carbohydrate antigen was prepared from cell walls of Streptococcus mutans strain MFe28 of monkey origin. The h antigen was extracted from the cell walls with 5% trichloracetic acid at 4 C, and purified by DEAE-Sephadex A-25 ion exchange chromatography followed by Sephacryl S-300 gel filtration. The purified antigen was composed of galactose (75%), glucose (16%), and rhamnose (3%). Although the antiserum against whole cells of S. mutans MFe28 gave a strong cross reaction with serotype d S. mutans, serotype h-specific antiserum could be obtained by adequate adsorption. The precipitin reactions and hapten inhibition test using serotype h-specific antiserum showed that galactose, glucose, and their derivative sugars were markedly potent inhibitors. It was concluded that the serotype h antigen is immunologically distinguishable from the known serotypes of S. mutans, although it is closely related to serotype d antigen of S. mutans.