Previous studies indicated the a antigen was widely distributed among strains of Streptococcus sanguis and the group H streptococci. The cell walls of strains containing this antigen had moderate to large quantities of rhamnose, small amounts of phosphorus, and little to no ribitol. The molar ratios of the peptidoglycan amino acids and hexosamines suggested a di-alanyl cross bridge. The homogeneity of the walls of these strains suggested that serological group H can be considered synonymous with S. sanguis. In contrast those strains that did not contain the a antigen had only small amounts of rhamnose in their cell walls, galactosamine and ribitol were always detected, and large quantitites of phosphorus were present. The molar ratios of peptidoglycan components in the latter strains suggested a direct alanyl-lysl cross bridge. Although many of these strains had been classified either as S. sanguis or group H streptococci because of minor serological cross-reactions and similar biochemical properties, the distinct differences between the composition of their cell walls and those of S. sanguis and group H streptococci indicate that they do not belong in this species or group. The cell walls of strains containing the a antigen are relatively homogeneous and therefore it is suggested that cells containing this antigen be considered Lancefield serological group H.