An exudate obtained from a dog, which contains the leukocytosis-promoting factor, induces a leukocytosis when injected into the peritoneal cavity of a guinea pig. Similar results are obtained with the recovered leuko-cytosis-promoting factor of exudates injected either intra-peritoneally or subcutaneously. The effect is quite specific. It fails to occur when other types of soluble materials are utilized. Consequently, the guinea pig presents itself as a suitable test animal for detecting the presence of the leuko-cytosis-promoting factor in inflammatory exudates, particularly if the material is injected into the subcutaneous tissue. Finally, as pointed out frequently in the past, these studies may have clinical application, for it is well known that the prognosis of an infectious process is to a large extent referable to the number of circulating leukocytes. For this reason studies have been initiated to determine the effectiveness of the leukocytosis-promoting factor on man. These studies are now in progress and will form the subject of a separate future communication in extenso. It can be pointed out, however, that four different patients with normal white cell counts have been injected intravenously with doses ranging from about 18 to 230 milligrams of active canine LPF. Within a few hours the white blood cell count increased without any change in temperature, and within several hours the count had risen 80 to 150%, showing that the material is both innocuous and potent on human beings.