A radioimmunoassay for the measurement of plasma progesterone is described. The 21-hemisuccinate of 11-desoxycortisol was coupled to human serum albumin using carbodiimide, and the conjugate was used to immunize 2 ewes. The antiserum obtained after 8 months of immunization from one of the ewes was used in the assay procedure. Incubation at 4 C overnight of this diluted antiserum was carried out in the presence of l,2-3 H-17-hydroxyprogesterone. Separation of free from bound hormone was achieved by a dextran-coated charcoal suspension. One ml of plasma or serum, with tritiated progesterone added for recovery, was extracted with ether, then chromatographed on a microcolumn of Celite. Recovery of 3 Hprogesterone after extraction and chromatography was 84.2+4.8% (SD). The sensitivity varied between 10 and 25 pg of progesterone. Recovery experiments of progesterone added to water and plasma confirmed accuracy. Concentration of progesterone in plasma from men averaged 233 ±64 (SD) pg/ml and from postmenopausal women 180 ±63 (SD) pg/ml. During the follicular and midluteal phases of the menstrual cycle, the levels of plasma progesterone were, respectively: 545 ±103 pg/ml (mean±SD) and 8561+4661 pg/ml. During pregnancy, the plasma level of progesterone varied with the stages of pregnancy, being 48.4 ±18 ng/ml (mean ±SD) at 16-18 weeks of gestation, 98.0 ±28 at 28-30 weeks and 178.5 ±48 at term. (J Clin Endocr 32: 619, 1971)
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