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Much evidence supports the concept that immunity to Group A streptococcal infection is type-specific; that is, that recovery from infection with one type of Group A streptococcus results in immunity to that type only. The M protein antigen in Group A streptococcal cell walls determines the serologic type specificity of various strains and is an important factor associated with strain virulence. Antibody to the M protein plays a fundamental role in immunity to Group A streptococcal infection and is, indeed, the only antibody known at present to confer significant protection against this organism (1-6).Despite the importance of anti-M antibody, extensive studies of its behavior in human streptococcal infections have been seriously hampered by the difficult and laborious biological methods heretofore available for its detection in human sera (7,8). Recently certain virulent strains of Group A streptococci were shown to form very long chains when grown in rabbit antisera containing homologous type anti-M antibody (9). This "long chain reaction" was found to be highly sensitive and type-specific.The present study is an evaluation of the "long chain reaction" as a biological test for type-specific antibody in sera of patients with streptococcal infections of known types. The results indicate that this readily demonstrable phenomenon correlates closely with the classical bactericidal test for anti-M antibody when appropriate indicator strains of streptococci are employed. Materials and MethodsStrains of Streptococd.--Several strains of virulent Group A streptococci representing a wide variety of serological types were screened for the property of long chain growth in the presence of homologous type rabbit antiserum.

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