Cathepsin B ( EC 3.4. 22.1) and Cathepsin L ( EC 3.4. 22.15) are cysteine endopeptidases implicated in the process of tumor invasion and metastasis, besides their numerous roles. This investigation was effectuated to study the activity levels of these two enzymes in human lung carcinoma tissues.Tissue samples were obtained from the Department of Thoracic and Cardiovascular Surgery immediately following surgery ( n = 15) and kept at -80°C till analysis. Tissue supernatants were obtained through extraction by repeated thawing and freezing (3x), followed by centrifugation at 17,200 g. For the determination of cathepsin B and cathepsin L activities in tissue extracts.the substrates "benzoyl -alanine- argininearginine-4-methoxy-2-naphthylamide" ( Z - Ala - Arg -Arg - MNA) and"benzoyl-phenylalanine-arginine-4-methoxy-2-naphthylamide" (Z - Phe - Arg - MNA) were used, respectively. Total soluble protein was determined by the Lowry procedure using bovine serum albumin as a standard. The specific activities were expressed as nanomoles min"'mg protein. The results obtained were as follows: 16.83 ±2.30 (Mean ±SEM) and 37.70 ±8.93 nanomoles min"1 mg protein for cathepsin B, and 8.73 ±1.64 (Mean ±SEM) and 17.58 ±5.05 nanomoles min'mgprotein for cathepsin L, for the normal and lung carcinoma tissues, respectively. Both Cathepsin B and L were determined to express higher activity in carcinoma tissues compared with normal tissues ( p = 0.035 and 0.042, respectively), favoring the role of these enzymes in malignant invasion.