The Lancefield's serotyping system of Streptococcus pyogenes is based on the M protein. The typing sera used for this system are prepared in rabbits immunized with whole organisms of specific serotypes. To remove cross reactive antibodies, the sera should be extensively absorbed with organisms of selected heterologous serotypes. In this study, the possibility of using monoclonal antibody (MAb) as a type specific reagent was discussed. MAb which specifically reacted to M type 12 protein of S. pyogenes were produced by cell fusion. This specific MAb reacted to hot acid extracted M type 12 protein in ELISA but didn't react in agglutination and precipitation. Latex beads were sensitized with MAb and examined by the coagglutination method. The latex reagent could detect a very small amount of M type 12 protein, so it could be used for M typing of S. pyogenes which produced a small amount of M 12 protein.