We may conclude from the results of the above experiments that no antiprolan is produced in the human organism during pregnancy. This has been ascertained in connection with the blood during the first as well as the last months of pregnancy, during delivery and one week postpartum. It is, therefore, evident that the organism does not use the antigonadotropic factor as a regulatory mechanism under physiological conditions. Antiprolan is exclusively elaborated in the organism after a type of prolan foreign to the species has been administered. Antiprolan formation must, therefore, be considered as a protective mechanism on pathological lines, as is similarly found in the immunity reactions, but not as a regulatory measure with reference to endogenous hormonal conditions. In accordance with this, no antiprolan has been proved to be present either in individual organs or in the blood of normal, amenorrhoic or sterile women or patients suffering from tumors.