A micro complement fixation assay was devised to measure the type-specificity of anti-M antibody. Unabsorbed sera were from rabbits hyperimmunized with heat-killed streptococci and from humans with naturally occurring antibody or immunized with purified M protein vaccines. These sera fixed complement only in the presence of homologous M proteins (serotypes 1, 3, 6, 12, and 14). The complement fixation reaction paralleled the results obtained with the Lancefield bactericidal (opsonic) assay and did not exhibit the cross-reactions frequently seen with type-specific anti-M sera assayed by passive hemagglutination. All serological activity of the antisera resided in the fraction corresponding to 7S globulin eluted from a Sephadex G-200 gel filtration column. Because of the close correlation between micro complement fixation and the bactericidal assay for type-specific antibody, it is proposed that the micro complement fixation procedure, as we have described it, merits further evaluation as a substitute for the bactericidal test of immunity to group A streptococcal infection
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