1. We begin with the discussion of the =mononuclear neutrophil cells=, Ehrlich's "=myelocytes=." They are present so abundantly in the blood of medullary leukaemia as to impart to the whole picture a predominantly mononuclear character. As we have frequently mentioned, myelocytes occur normally only in the bone-marrow, not in the circulating blood. Their eminent importance for the diagnosis of myelogenic leukaemia, where they have been regularly found by the best observers, is in no way diminished by their transitory appearance in a few other conditions (see pages 77, 78). Though they have been occasionally found, according to Tuerk's investigations, in the critical period of pneumonia as parts of a general leucocytosis, the danger of confusion with leukaemic blood changes is non-existent. This is guarded against by (1) the much smaller increase of the white cells; (2) the diminution of the eosinophil and mast cells; (3) the fact, that the myelocytes of leukaemic blood are nearly always considerably larger; (4) the preponderating polynuclear character of the leucocytosis, which is not effaced by the small percentage amount of myelocytes (at most 12%): (5) the incomparably smaller absolute number of myelocytes. In the most pronounced case of Tuerk's, for example, in which the percentage number of myelocytes amounted to 11.9, calculation of their absolute number gives at most 1000 myelocytes per mm.^{3} This is a figure which bears no comparison with that obtaining in leukaemia, where 50,000-100,000 myelocytes per mm.^{3} and over occur in cases that are in no way extreme. 2. =The mononuclear eosinophil cells.= Before the introduction of the staining method, Mosler had described large, coarsely granulated cells, "marrow cells," as characteristic for myelogenic leukaemia. These are to be regarded as for the most part identical with the mononuclear eosinophil cells, noticed by Mueller and Rieder as peculiar, and aptly described by them as the eosinophil analogues of the preceding group. They appear as large elements with oval, feebly staining nucleus. Undeniably a valuable sign of leukaemia, they are not nearly so important as the mononuclear neutrophil cells, as follows from the numerical superiority of the latter. To regard the presence of "eosinophil myelocytes" as absolute proof of the existence of a leukaemia is inadmissible, since they are occasionally present in small numbers in other diseases.