Summary: The bactericidin active against B. subtilis has been quantitated in the serum of the rat, rabbit, human, guinea pig, cow and dog. High values were obtained only in the rat and rabbit. Although the serums of healthy humans contain only small amounts of bactericidin the levels may increase several fold during the acute phase of various illnesses. The bactericidin of rabbit serum was studied in detail. However, its tissue source was not determined and all attempts to extract bactericidin from the spleens, livers, polymorphonuclear leukocytes and monocytes of rabbits were unsuccessful. Bactericidin in rabbit serum pH 7.4 was found to be completely destroyed by heating at 60°C for 2 hours and is a substance of cationic nature readily absorbable with bentonite. Its inactivation by citrate is apparently due to its property of forming a salt with citrate rather than to the withdrawal of calcium ions by citrate. Electron microscope studies of the comparative cytologic effects of lysozyme and bactericidin on B. subtilis indicated that lysozyme exerts its principal lytic effect on the cell wall, whereas bactericidin tends to lyse all components of the cell. It is also notable that polymorphonuclear leukocytes which are rich in lysozyme do not appear to contain bactericidin. The results provide strong evidence that the bactericidin of rabbit serum, active against B. subtilis, is distinct from lysozyme and independent of lysozyme in its action.