A technique has been developed for collecting large numbers of polymorphonuclear leucocytes from peritoneal exudates in rabbits. These cells are obtained essentially free from other cell types and from debris. When microphages so procured are disrupted by physical methods and extracted with aqueous salt solutions, the soluble fraction manifests striking bactericidal activity, especially on Gram-negative enteric bacilli. The susceptible microorganisms are not lysed. This bactericidal substance, which has been called phagocytin, appears to be limited in distribution mainly to the polymorphonuclear leucocyte. No phagocytin is present in extracts of rabbit heart, kidney, or skeletal muscle, and rabbit liver and spleen contain much less than do packed leucocytes. Extracts of human and of guinea pig microphages show less bactericidal activity than rabbit cell preparations. Similar extracts of rat and mouse polymorphonuclear leucocytes contain no demonstrable phagocytin. As indicated by its behavior on dialysis, on exposure to proteolytic enzymes, and on salt fractionation, phagocytin appears to be a protein with general properties characteristic of a globulin. It is clearly different from lysozyme and from properdin. Although phagocytin is reasonably stable at temperatures of 65 C. and lower for several hours, solutions of it gradually lose bactericidal activity on standing for prolonged periods at 4 C. This instability, and also the ease with which phagocytin is inactivated, presumably by adsorption, on exposure to a variety of materials, have thus far rendered fruitless efforts to isolate it.