Although the blood picture of myelogenic leukaemia has been so clearly drawn by Ehrlich, misconceptions and obscurities still occur in the literature. And they are due to great errors in observation. It has for instance happened that unskilled observers have regarded and worked up cases of lymphatic leukaemia as myelogenic. The apparent deviations discovered in this manner are copied, as specially remarkable, from one book to another. Through insufficient mastery of the staining method, the characteristic and diagnostically decisive elements (neutrophil myelocytes for example) are frequently mistaken. A further source productive of misconceptions lies in the circumstance that the typical leukaemic condition of the blood may essentially change under the influences of intercurrent diseases. Thus the intrusion of a leucocytosis, brought about by secondary infection, is able to obliterate more or less the specific character of the blood. Such conditions must naturally be considered apart, and should not be used to overthrow the general characteristics of the picture. No one surely would deny the diagnostic value of glycosuria for diabetes, because in conditions of inanition, for instance, the sugar of a diabetic may completely vanish, although the disease continues. And one does not deny the diagnostic value of the splenic tumour in typhoid fever, because the enlargement of the spleen may occasionally subside, under the influence of an intestinal haemorrhage. From these considerations it is obviously necessary to derive the description of leukaemic blood from pure uncomplicated cases; and to construct it with the aid of standard methods. In this manner a type is obtained so characteristic, as to render diagnosis absolutely certain from the blood alone.