Strains of type 6 (S 43) and type 14 group A streptococci were grown with M-protein production in the presence of chemically defined synthetic media slightly modified from that previously employed for the growth of a nonproducer of M protein (type 4). The M protein, which is associated with virulence in group A streptococcus, was previously produced in growing cultures only with complex media. The bacterial growth with the biosynthesis of M protein in synthetic medium was obtained by successive adaptation in steady-state culture with decreasing amounts of Todd-Hewitt broth. The synthesis continued for at least 480 generations at pH 7.3 and with a generation time of 84 min. Glucose was the limiting nutrilite and the concentration of reducing agents in the medium was critical. The M protein was identified by gel diffusion against type-specific antisera from the Communicable Disease Center and from R. Lancefield. The yield of M protein obtained from organisms grown in the continuous-culture device was comparable to that from standard broth stationary cultures.