The successful classification of Group A streptococci by the capillary precipitin technique requires a complete series of M type antisera which are sufficiently potent and specific to give unequivocal type-specific reactions with all the serotypes. Specific antisera for this purpose have been prepared by absorption with heterologous streptococci. Unabsorbed antisera have been employed here in the Ouchterlony double-diffusion agar-gel test to identify the M type of streptococci. Techniques have been developed for making this method of M typing fully reliable. The results reported here confirm and amplify the original findings of Michael and Massell (3). With crude HCl extracts and unabsorbed M type antisera, a precipitin line due to the M protein and another to the group-specific carbohydrate are the two major reactions observed. These reactions, however, are usually readily distinguishable. There was a surprising lack of cross-reactive precipitin lines due to non-type-specific protein antigens in the extracts. Although many of the unabsorbed M type antisera can be employed in the double-diffusion tests, the group-specific antibody must be removed from some of the unabsorbed antisera to avoid confusing cross-reactions. Absorption of these antibodies has been achieved by means of a specific immunoabsorbent column prepared from para-aminophenyl-beta-N-acetylglucosamine and cyanogen bromide-activated Sepharose. Excellent agreement was observed between the M typing results obtained on 117 field strains by the conventional capillary precipitin method and the Ouchterlony double-diffusion method.