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We describe a rapid and direct 125I-based radioimmunoassay for quantification of unconjugated (free) estriol in pregnancy serum. Estriol in serum is adsorbed onto a small column of Sephadex, thereby allowing its separation from proteins and interfering materials. A radioiodinated derivative of estriol (6-ketoestriol-6(o-carboxymethyl)oxime-[125I] tyrosine methyl ester) is added to the column, followed by a limiting amount of antiserum. After incubation, the antibody-bound hormone is separated by a buffer wash. The assay exhibits satisfactory recovery and parallelism, and the intraand inter-assay coefficients of variation are <10%. The values obtained by using the assay correlate well (r = 0.97) with those from a comparison method in which solvent extraction and chromatographic purification are used.

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