Background : Malignant skin tumor cells derived from epidemal keratinocyte penetrate the basement membrane to proliferate in dermal interstitial strozirarnd invade surrounding tissue and finally metastasize to distant organs. In this stage of invaiac a and metastsis, the existence of proteolytic enzymes, which are capable of degrading the tissue barrier composed primarily of collagen, elastin, glycoproteins and proteoglycans, is imagnant. One of theses enzymes, cathepsin B, a lysosomal thiol protease, has been reported to ie ound in association with plasma membrane in animal and human tumors and to be releasec b tumor cells. Object : We assayed the cathepsin B activities of squamo is cell carcinoma and basal cell carcinoma in order to investigate the correlation between the degree of cathepsin B activities and invasiveness or metastatic potential of skin tumors. Methods : Cathepsin B-like protease activity was measurec by the method of Hirao using a synthetic substrate, Z-Phe-Arg-MCA. The skin tissues (penie Koreskin for control, basal cell carcinoma and squamous cell carcinoma tumor masses) were homogenized and their subcellular organelles were fractionated by centrifugation. Each of the fractionated preparations were used as enzyme solution. Results : Cathepsin B-lilke activities were found mainly in the membrane fractions in all the samples. The activities of squamous cell carcinoma (12.484+1.904) and basal cell carcinoma (10.598+1.926) were higber than those of the control skin (9.115+0.815). Conclusion : These results suggest that membrane-bound conrt epsin B-like protease participates in local dissolution of the extracellular matrices and were ar endothelial cells to be able to make metastasis to other remote organ during the invasivest ges of malignant skin tumors. (Kor J Dermatol 1994; 32(6): 971-976)