Three different types of neutral proteases related to collagen metabolism have been found in the granule fraction of human leucocytes from normal adults, using collagen, gelatin, and synthetic peptides as substrates. These are collagenase, an enzyme showing a potent hy-drolytic activity against gelatin but little against native collagen, and one splitting the crosslinks region of collagen. Their molecular weights were estimated to be about 75,000 150,000, and 25,000, respectively, by gel chromatography. The former two enzymes were inhibited by a α2-macroglobulin and ethylenediaminetetra-acetate, but not by α1-proteinase inhibitor (α1-antitrypsin) or phenylmethylsulfonylfluoride, while the latter enzyme, associated in behavior with an enzyme hydrolyzing succinyl-(L-alanyl)3-p-nitroanilide, was inhibited by α1-proteinase inhibitor, α2-macroglobulin, and phenylmethylsulfonylfluoride, but not by ethylenediaminetetraacetate. A possible cooperative function of these enzymes in collagen catabolism is discussed.