A simple radioligand assay has been developed for measuring cortisol in plasma and urine. The antiserum was produced in rabbits by using cortisol-21-hemisuccinyl-bovine serum albumin as the antigen. The standard curve was linear from 0-1000 pg. A small aliquot of ethanol-precipitated plasma or urine is directly assayed. Interassay variability of 56 separate assays of a normal plasma pool during six months was: mean, 224 µg/liter; range, 210-238 µg/liter; CV, 3.2%. In assay of multiple samples obtained between 0800 and 1800 hours from normal young men, radioimmunoassay (RlA) gave consistently lower values than did competitive protein binding (CPB). The relationship, as calculated from results for 41 single samples, is expressed by the equation RIA = 0.713 CPB + 0.59, suggesting that the normal range for RIA should be 35-180 µg/liter rather than the 50-250 µg/liter usually cited. Although there is crossreactivity with 11-deoxycortisol, this assay is clearly superior to CPB: it is more specific, has a wider range (<8-400 µg/liter), and requires less technician time.

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