Strains of group A streptococci, types 30 and 12, were observed to grow in extremely long chains in liquid media to which homologous antiserum was added. Addition to the media of antisera to heterologous types of streptococci failed to produce long chain growth. The long chain effect was destroyed by absorption of the antiserum with organisms of homologous type but was unaffected by absorption with organisms of heterologous types.

The reaction disappeared at concentrations of antisera smaller than 0.25 per cent and was independent of complement or other heat-labile serum factors. Addition of trypsin to the culture to remove M protein from cells prevented long chain formation. The long chain effect depended upon the constant presence of antibody to the media. In its absence, the organisms promptly reverted to short chain growth.

The phenomenon appears to have general applicability to those strains rich in M protein with only an occasional strain not responding as described. Further studies are in progress to determine the cause of this atypical response. The applicability of this phenomenon in detecting type-specific antibody using indicator strains of a variety of streptococcal types is discussed.

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