Cathepsin L is a lysosomal endopeptidase belonging to the papain cysteine protease superfamily. It is synthesized as pre-proenzyme and after posttranslational modifications and its transport to the lysosomes cathepsin L is participating in the bullk lysosomal protein breakdown or in special cellular functions such as antigen pro-cessing. Certain specialized cells like macrophages and osteoclasts secrete the precursor of cathepsin L, procathepsin L, which can easily be processed to some form of active enzyme by acid or surface activation or limited proteolysis and which then is involved in connective tissue degradation and the turnover of extracellular matrix proteins. Like other lysosomal proteinases procathepsin L was also found to be secreted by many malignantly transformed cells and the level of its mRNA and its protein expression and the extent to which it is secreted seem to be correlated to the malignant potential of such cells. Since cathepsin L is capable of degrading protein constituents of the extracellular matrix it is assumed to play a crucial role in tumor progression and metastasis and a number of other disorders where the destruction of the extracellular matrix is the major cause of disease (e.g. rheumatoid arthritis, neurodegeneration). Inhibition of the enzyme or the proenzyme by low molecular weight inhibitions or antibodies in in vitro and in vivo test systems led to a suppression of the invasive capabilities of malignant cells or a decline in their ability to form tumors. Cathepsin L is closely related to the recently described cathepsin V (L2) and the sequence indentity of almost 80% between them requires antibodies for the detection of cathepsin L which do not crossreact with cathepsin V. This requirement can only be met by epitope specific monoclonal antibodies as are used in RBMS257R. Elevated levels of cathepsin L have been found in primary cell cultures from cancerous prostatic samples. Higher levels of active cathepsin L were found in breast tumor cells that are invasive, compared to those that are not invasive. Enforced expression of the precursor procathepsin L was shown to confer high tumorigenic and metastatic properties to human melanoma cells. The upregulation of cathepsin L and B by ros Oncogene has been reported for primary human calorectal carcinomas. Higher levels of cathepsin L were found in rheumatoid arthritis synovial lining compared with osteoarthritis. Cathepsin L activity has been described in aortic aneurysm wall and parietal thrombus acting on proteins determing elasticity and mechanical resistance of arberies. Cathepsin L was found to be necessary for Li (invariant chain) degradation, a critical step in major histocompatibility complex class II – restricted antigen presentation, in cortical thymic epithelial cells, but not in bone marrow derived antigen – presenting cells.