THE history of the discovery of insulin occupies an almost unique place in the scientific literature of recent times, and in the supplement which accompanies this issue we print an account of the present position of investigations regarding this substance. So many investigators had previously been within an ace of making its discovery that, as Prof. MacLean says, “it still remains a mystery why insulin was not isolated many years ago.” The discovery is a good illustration of two salient facts bearing on scientific research work: first, that a very slight modification of a technique which had led previous explorers to failure might lead one to success; secondly, that research work of an applied nature, such as the search for a cure for diabetes, is very closely dependent upon related investigations which belong more properly to the domain of academic science-in this instance the improvement of methods for the accurate determination of small quantities of glucose in the blood. Insulin as now placed on the market is a therapeutic-ally trustworthy preparation, and there must be many persons who owe life and health to the careful administration of this substance. But we must not be too sanguine that a diabetic taking insulin is to be regarded as a normal person. Prof. MacLean's article also contains a warning which it is to be hoped will be taken seriously to heart: careful investigation has shown that no beneficial results ensue from taking any pancreas preparation by the mouth, and this also applies to insulin itself, which is speedily destroyed by the juices present in the alimentary canal. From the academic point of view, insulin presents many problems of absorbing interest. At present the chief of these is the problem of what happens to the blood sugar and to glycogen after the administration of insulin: both apparently disappear, yet no inter-mediary or end products have yet been traced. Lastly, it cannot be too strongly emphasised that the discovery, from start to finish, could not have been made without those experiments on living dogs which some would seek to have abolished.