A microtiter complement fixation (CF) procedure was developed for use in detection of antibodies in sera of swine and rabbits vaccinated with Streptococcus equisimilis. Crude ultrasonic as well as acid-extracted preparations contained CF antigen, but the ultrasonic procedure resulted in a higher yield of active antigen. Evidence of serotype specificity with varying degrees of cross reactivity was detected with the CF procedure when representative strains of four different serotypes of S. equisimilis were compared by using their respective unadsorbed antisera. Fractionation of crude sonic extract or acid extract by centrifugation, precipitation with ammonium sulfate, and chromatography on carboxymethyl cellulose yielded a purified, type-specific antigen that reacted only with the homologous antiserum in the CF test and formed a single band by immunodiffusion. Complement-fixing antibodies in immune swine sera were predominately immunoglobulin G.
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