1. A new highly sensitive and accurate ultramicro method for the estimation of testosterone in human peripheral plasma is described. The method uses paper-and thin-layer-chromatographic separation of plasma testosterone, which is determined as testosterone diheptafluorobutyrate by electron-capture detection after gas–liquid chromatography. 2. The average difference between duplicates is ±2% (range 1–5%) with as little as 2·5ml. of human male peripheral plasma. With 10ml. of plasma the method is sensitive enough for the accurate determination of testosterone in human female plasma. The high order of accuracy is achieved by the use of a radioactive label and an internal standard for gas chromatography, and by obtaining several gas chromatograms from the same plasma sample. 3. As little as 40μμg. of peripheral plasma testosterone can be detected. The method is 20 times as sensitive as electron-capture techniques with the monochloroacetate derivative. 4. The method is simpler and quicker than double-isotope-derivative methods, and slightly more sensitive. The advantages of the method, which is specific for testosterone, are its high sensitivity and accuracy, which are achieved with relative convenience.
