IN a recent communication (Proc. Physiol. Soc., October 1930) Thompson demonstrated that by certain methods of extraction a growth inhibiting substance could be obtained from fresh parathyroid glands. Further work in this laboratory has shown that a similar substance can be extracted from the flesh of vertebrates. Purification of such extracts shows that the substance responsible is thermo-stable, is not a sterol, and is probably nitrogenous.