COAGULASE production is a property of all pathogenic Staphylococci and is the most commonly accepted single criterion of pathogenicity. In addition it has been shown that coagulase plays a part in staphylococcal infections. Evidence for this is indirect: first, clottable plasma inhibits phagocytosis of Staphylococci1; secondly, rabbits can be protected from Staph. aureus infection by human coagulase-inhibiting serum2, and, also, they receive a degree of immunity against such infection by active immunization with coagulase3.