UP to, and for some time after, the discovery of insulin it was tacitly assumed that all cases of human diabetes could be explained by deficiency of insulin, but during the last decade suspicions have arisen regarding the correctness of this assumption. These suspicions had their origin in clinical experience which compelled the realisation that the efficiency with which insulin acts in different patients, and in the same patient in different states of health, varies considerably. This disclosed the possibility that a state of diabetes might result from inefficient action of insulin as well as from lack of insulin.