Cathepsin L (catL) is a cysteine protease of the papain family. The aim of the present study was to investigate the functional role of cathepsin L in the human lung carcinoma cell line A549. Therefore, catL antisense transfected A549 clones were generated. They showed decreased catL-specific mRNA levels, protein concentrations and proteolytical activities. Functional analyses revealed a decreased growth rate associated with an increased sensitivity to proapoptotic agents. Additionally, we observed in catL-deficient A549 cells an increased migration rate as well as an enhanced invasion into matrigel. Using casein as an unspecific substrate we found an increased proteolytic activity. Further analyses revealed an increased cathepsin D activity and protein expression. Recently, it was shown that cathepsin D may be involved in the regulation of apoptosis as well as in matrix destruction. Furthermore, the catL-deficient A549 cells showed an 5-10-fold enhanced IL-8 secretion in comparison to control cells, which may be another explanation for the increased migration. The production of IL-6, IL-18 and TGF-ß1/2 was not affected, whereas the MCP-1 production was rather decreased. In conclusion, the data suggest that cathepsin L may be involved in the regulation of cytokine production and cathepsin D expression thereby affecting the induction of apoptosis of the A549 cells.