The treatment of diseased conditions by means of induced pyrexia is now so frequently carried out that it is surprising how little we know of the biological processes involved. It is not an exaggeration to say that the beneficial effects of malaria in general paralysis are as gratifying as those of liver in pernicious anæmia, and of insulin in diabetes, but we still await a satisfactory explanation of its mode of action. An attempt in this direction was made by Eddison (1), who observed a failure of leucopoiesis in untreated cases of paralytic dementia, which he attributed to degeneration in the reticulo-endothelial system. He considered that, in general, pyrexia was associated with leucocytic reactions, and that this resulted indirectly in reticulo-endothelial stimulation. It is not proposed to consider here the arguments put forward by Eddison, but he should be given credit for being the first person in this country to draw attention to a very important aspect of pyretotherapy.