Certain strains of Group A streptococci showed striking increase in chain length when grown in liquid media to which was added human sera that contained antibody to M protein of homologous type. This "long chain reaction" was shown to be a highly specific and sensitive biological test for human type-specific antibody and correlated closely with the classical bactericidal test.

Patients infected with Type 12 or Type 3 Group A streptococci showed the appearance of anti-M antibody in their sera by both methods at similar intervals during convalescence. Of 217 sera studied in these patients the two tests showed agreement in all but 11 specimens. Of 99 patients who were bled serially following Type 12 or Type 3 infections, and whose sera were tested by both methods, there was close agreement, the bactericidal test being only slightly more sensitive.

The advantages and limitations of this new biological test for human type-specific immunity are discussed.

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