SUMMARYA radioimmunoassay for the determination of testosterone in human plasma is described. After extraction from plasma, testosterone is separated by paper chromatography. The radioimmunoassay is performed using an antiserum to testosterone‐3‐oxime‐rabbit serum albumin and a saturated solution of ammonium sulphate is used for separation. The reliability criteria of the method in terms of precision, accuracy, sensitivity and specificity have been evaluated.The mean level of testosterone in plasma samples from nineteen normal men (age range 16–73 years) is 527±226 (±SD) ng/100 ml. In ten normal ambulatory females (in the second phase of the cycle) aged 19–35 years the mean plasma testosterone is 27±8±10±6 (±SD) ng/100 ml. In twenty‐four impotent men (aged 18–50) the testosterone level is 431±191 (mean±SD) ng/100 ml. These values are not significantly different from normal subjects.Lastly, the testosterone levels found in a few cases of male hypo