We have previously cloned a rabbit cDNA clone (OC-2) from an osteoclast cDNA library by the differential screening. OC-2 was found to encode a novel cysteine proteinase, tentatively called cathepsin K, which is predominantly expressed in osteoclasts. By use of a rabbit OC-2 fragment as a probe, its human counterpart was cloned from a cDNA library of osteoarthritic hip bone. The cloned human cDNA (hOC-2) encoded a protein of 329 amino acid residues and its deduced amino acid sequence showed 94% homology to rabbit cathepsin K. Multiple alignment of amino acid sequences of human cathepsins B, H, L, S and K showed the highest homology of cathepsin K to cathepsin S 48%. Northern blot analysis showed that cathepsin K mRNA is expressed at high levels in some osteoarthritic hip bones and at a very high level in osteoclastoma compared to very low levels in other tissues. These results suggest that cathepsin K is closely involved in human osteoclastic bone resorption.