Abstract Extracts of human articular cartilage contain a variety of inhibitors to serine, cysteine and metallo-proteinases. By gel filtration chromatography, inhibitory activity towards serine proteinases was resolved into two components of apparent molecular weights 62,000 and 12,000 daltons; whereas inhibitory activity towards cysteine proteinases eluted with an apparent molecular weight of 13,000 daltons. In both cases the low molecular weight inhibitors were further resolved into two components by ion-exchange chromatography. Inhibitory activity towards metallo-proteinases resolved into two components of apparent molecular weights 35,000 and 25,000 daltons. No inhibitor of aspartic proteinases was detected. Although most of the inhibitory activities to serine and cysteine proteinases could be extracted from cartilage with 1 M NaCl, the complete removal of metalloproteinase inhibitory activities required extraction with 4 M guanidinium chloride. This suggests that they are more strongly associated with the cartilage.