The effect of hydrogen peroxide (H 2O 2) and heat treatment has been studied as affecting the biological activity and the immunological specificity of the gonadotropins of three species. The hormones studied were human chorionic gonadotropin (HCG), human menopausal gonadotropin (HMG), pregnant mare serum gonadotropin (PMSG), follicle-stimulating and luteinizing hormones (FSH and LH) of human and ovine origin. The results indicated that heating at 100° for 30 min completely destroyed the follicle-stimulating (FS) activity present in HCG, HMG, and human and ovine FSH. The FS principle in PMSG was destroyed at 70° and the LH activity at 100°. Human and ovine LH were not significantly affected by this treatment. The immunological activity of the follicle stimulating principle present in HCG, HMG, and human FSH was completely destroyed on heating the hormones at 100° whereas the luteinizing principle in HCG, human LH, and PMSG was only partially affected. The activities of both ovine FSH and LH remained unaffected. Treatment with 30% hydrogen peroxide for 1 hour at room temperature destroyed completely the biological activity of the luteinizing principle in HCG and HMG, human LH, and ovine FSH. The luteinizing principle in ovine LH was partially destroyed by this treatment, but the follicle-stimulating activity of human FSH, HCG, and HMG remained unaffected. Treatment with 30% hydrogen peroxide failed to destroy the immunological activity of human and ovine FSH and ovine LH, but completely destroyed the LH activity present in HCG, HMG, and human LH. The follicle-stimulating principle in gonadotropins of the human species is inactivated when the hormones are heated at 100° for 30 min whereas the LH principle is destroyed by treatment with 30% hydrogen peroxide. There is, however, a difference in the selective inactivation of the follicle-stimulating and luteinizing principles of gonadotropins of the other species studied in spite of the similarity in their biological actions.