The inhibiting action of the blood serum upon the enzyme of the polynuclear leucocytes, leucoprotease, is exerted by the albumin fraction of the serum. The albumin fraction contains no proteolytic enzymes. The globulin fraction of the serum contains no anti-enzyme for leucoprotease; it contains, on the contrary, an enzyme which digests proteids in a neutral or alkaline medium. This enzyme resembles leucoprotease which is present in the polynuclear leucocytes of an inflammatory exudate and in the bone marrow from which these cells are derived, and is doubtless identical with the similar enzyme occurring in smaller quantity in the spleen. This enzyme which is present in the blood serum is held in check by its anti-enzyme, but the latter is in such excess that the serum as a whole is capable of checking the action of leucoprotease when added in considerable quantity. Leucoprotease of one mammalian species is inhibited by sera of other mammalian species, but the anti-enzymotic activity of various sera differs; the anti-enzyme of the rabbit's serum is stronger than that of dog's serum, when tested either with dog's or with rabbit's leucoprotease. The co-existence in the rabbit of leucoprotease with feeble strength and anti-body of great activity may explain the absence in these animals of typical suppuration with liquefaction of tissues. The serum of birds which have been tested, namely, pigeon and hen, almost completely fails to inhibit mammalian leucoprotease (of dog). The polynuclear leucocytes, the bone marrow and the spleen of the hen do not contain an enzyme resembling leucoprotease of mammals. The absence of anti-enzyme in the serum is associated with absence of a corresponding enzyme in the leucocytes.