SUMMARY Extracts of human plasma in ethanolic sulphuric acid contain two groups of fluorogens. The fluorescence of one of these increases linearly with time, while the other gives a constant fluorescence between 8 and 20 min. after extraction into the acid mixture. Measurement of the rate of increase in fluorescence of the plasma extracts permits the proportion of the total fluorescence due to the stable component to be calculated. The component with constant fluorescence probably consists entirely of cortisol and corticosterone. This finding has been used to improve the specificity of a simple method for the determination of adrenal corticosteroids in human plasma. This improved method is described and assessed, and a range of normal values is reported.

SUMMARY A technique for the bioassay of adrenocorticotrophic hormone (ACTH) in human plasma is described, and its experimental validation discussed. Segments of guinea‐pig adrenal are maintained individually for 5 hr in nonproliferative organ culture in a medium reinforced with 10‐3m sodium ascorbate. At the end of this time each segment is exposed to either standard or unknown ACTH for 3 min. The segments are then rapidly chilled to ‐ 70oC and sections cut in a cryostat. The sections are stained by the ferric ferrocyanide technique for reducing groups, and the degree of staining in the zona reticularis measured by scanning and integrating micro‐densitometry. There is an inverse linear correlation between the intensity of the stain and the logarithmic concentration of ACTH over the concentration 0.0025‐2.5 pg/ml of culture medium. The assay is precise (λ= 0.09) and extremely sensitive. Concentrations of ACTH down to 1 pg/ml may be determined on as little as 0.1 ml of plasma without extraction. Results obtained are in a range similar to that given by the Lipscomb‐Nelson bioassay and by radioimmunoassay.

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