The capsular polysaccharide of Haemophilus influenzae type b was intrinsically labeled with tritium by a microculture technique with 6-3H-d-glucose and was isolated in radioantigenically pure form by a combination of selective precipitation and molecular sieve chromatography. Labeling with tritated sugar residues approached one-fourth maximum and produced a specific activity 10-fold that previously described for extrinsic labeling methods. In radioantigen-binding assays for antibody, sensitivity depended on the size of the antigen; preparations were readily made that could detect 0.01 µg Ab/ml in serum samples of 25 µl. Stability of the labeled antigen appears limited only by the primary radiodecomposition of tritium.

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