Comparison of standard curves with different concentrations of human plasma in the medium of incubation is presented. It appears that Bo varies with the dilution although the ratio B/Bo is independent of the dilution. This last observation was used to research ‘endogenous’ LH-RH in four menstrual cycles, in postmenopausal women and in men. The validity of the dosage is discussed in a more general way in the light of the results published in the literature.