Two latent collagenases whose apparent MW were .apprx. 150,000 and 60,000 were detected in human leukocytes and the partial purification of the high MW component was accomplished by the following steps: acetone precipitation, ammonium sulfate fractionation, gel filtration on Sephacryl S-200, chromatography on DEAE-Sepharose and a final gel filtration on Sephacryl S-200. After activation by an activator extracted from human rheumatoid synovial fluid, the enzyme was able to cleave collagen into the classical 1/4 and 3/4 fragments and was inhibited by chelating agents and other typical collagenase inhibitors.