A radioimmunoassay was developed for the quantitation of antibodies to the Group A, A-variant, and C carbohydrates in rabbit streptococcal antisera. The assay employs radiolabeled purified Group carbohydrate which has been tyrosylated to allow for incorporation of 125I. This assay has an advantage over the quantitative precipitin test because it measures non-precipitating antibody in addition to precipitating antibody. Furthermore, 7S anti-IgG in rabbit antisera give a falsely elevated value for the antibody concentration in the quantitative precipitin test. This did not occur with the radioimmunoassay. The assay described is reproducible, sensitive, and uses little antisera.

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This work was supported in part by National Institutes of Health Grant AI08429 and conducted under the sponsorship of the Commission on Streptococcal and Staphylococcal Diseases, Armed Forces Epidemiological Board, and was supported by the Office of The Surgeon General, Department of the Army, Washington, District of Columbia.

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