The problem of leucocytosis has been the subject of much discussion in recent years. Amongst the earlier workers on the subject Virchow stands prominent, and he first gave the name of leucocytosis to a temporary increase in the number of leucocytes in the blood, this occurring both in physiological and pathological conditions. During the past twenty years special attention has been paid to this phenomenon, bringing to light some very important information. Amongst later workers Metchnikoff has done more to enlighten us as to the problem than any other worker. To briefly recapitulate his doctrine. The leucocytes protect the organism against harmful germs by catching them up in their pseudopods, by investing them, and thus robbing them of the possibility of exerting their deleterious action externally. The termination of an infective process would therefore depend alone on whether leucocytes possessing this function are present in the blood in sufficient numbers to overcome the invasion of the germs. The doctrine of Metchnikoff has been modified and also extended by other workers, notably Denys, Löwy, and Richter, who have proved that the value of the leucocytes does not depend on their pseudopods, but that their chemical products yield the strongest protection to the organism. The leucocytes are able by means of the bactericidal or antitoxic substances which they give off to paralyse the toxins produced by the bacteria, and in this way render the microbes harmless by depriving them of the weapons of attack, even if they cannot destroy them.