A method has been described for the preparation of a potent antibacterial factor from rabbit polymorphonuclear leucocytes. Upon characterization, the factor was found to possess many properties in common with basic proteins. The amino acid analysis revealed that it contained a relatively large amount of arginine (17 per cent) and small amounts of the other two basic amino acids. It has therefore been identified as a protamine or protamine derivative.

The leucocyte factor was very active against all Gram-positive pathogens tested but exhibited little or no action against Gram-negative species. A possible explanation of this phenomenon has been discussed. The factor was very heat-stable at acid and neutral pH and its staphylococcidal activity was blocked by glutamyl polypeptide, hyaluronic acid, and desoxyribonudeic acid.

Because of the apparent similarity of the product studied here to other poorly defined leucocyte factors which had been termed leukins in the early literature, it is suggested that the name leukin be retained for it. The possible significance of this leukin in natural immunity has been discussed.

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