1. The increase of polynuclear cells occurring in infectious processes, often called inflammatory, after the principle "_a potiori fit denominatio_." The majority of febrile infectious diseases, pneumonia, erysipelas, diphtheria, septic conditions of the most varied aetiology, parotitis, acute articular rheumatism, etc. are accompanied by a leucocytosis of greater or less extent. In this connection uncomplicated typhoid fever and measles occupy a peculiar position. In them the absolute number of white blood corpuscles is diminished, and chiefly at the expense of the polynuclear neutrophil cells. For the details we have quoted, and for the course and variations of leucocytosis in infectious diseases we refer to the thorough monograph of Tuerk. Of Tuerk's observations we will mention only that in the final stage of the process of leucocytosis, which occurs at the time of the crisis in diseases which run their course critically, mononuclear neutrophil cells and stimulation forms as well often make their appearance in the blood. In still later stages, in which the blood has once more a nearly normal composition, a moderate increase of the eosinophils--gradually waxing and again waning--is very frequently found (Zappert and others). Stienon, who has likewise devoted special researches to the occurrence of leucocytosis in infectious diseases, shews this point very well in his curves. 2. =Toxic leucocytosis= occurring in intoxications with the so-called blood poisons. This important group has not yet received adequate treatment in the literature. In general the majority of blood poisons, potassium chlorate, the derivatives of phenyl hydrazin, pyrodin, phenacetin call forth even in man a considerable increase of the leucocytes besides the destruction of the red blood corpuscles. This has been observed experimentally by Rieder. We observed marked increase of the white blood corpuscles after poisoning from arsenurietted hydrogen, from potassium chlorate, further in a fatally ending case of haemoglobinuria (sulphonal poisoning?) as well as after protracted chloroform narcosis. 3. The =leucocytosis= which accompanies acute and chronic anaemic conditions, especially posthaemorrhagic. 4. =Cachectic leucocytosis= in malignant tumours, phthisis, etc.[26] To enter here more precisely into the special clinical importance of blood investigation in different forms of disease would lead us too far, and we refer for this subject to the excellent and thorough monograph on leucocytosis by Rieder and to the papers of Zappert and Tuerk. In this place we will only touch on the most weighty points. [alpha]. The importance for differential diagnosis of the leukopenic blood condition in typhoid fever as compared with other infectious diseases, and in measles as against scarlet fever.