SUMMARY: 1. A method for the detection and estimation of gonadotrophins in the plasma of nonpregnant human subjects is described. 2. The end-point of the bioassay is the mouse uterus test, and results can be expressed in terms of a reference standard (HMG-20A) prepared from the urine of menopausal and postmenopausal women. 3. The method has been found satisfactory for estimations in postmenopausal women and in other conditions in which the concentration of gonadotrophins in plasma is relatively high. 4. The reliability criteria for the method are discussed under the headings of accuracy, precision, sensitivity and specificity. 5. Interstitial cell stimulating hormone activity, as determined by the hypophysectomized rat prostate test, and follicle stimulating hormone activity, as determined by the ' HCG augmentation' test, were both detected in human plasma. 6. A preliminary series of results in individual subjects is reported.