The major excreted protein (MEP) of malignantly transformed mouse fibroblasts is a secreted thiol proteinase. Sequencing of the MEP cDNA shows the coding region for the protein to be identical with the sequence for a mouse cysteine proteinase isolated from macrophages, but the MEP cDNA is polyadenylated at a different site in the 3' non-coding region. Strong homology of MEP with human cathepsin L suggests that MEP is the mouse analogue of cathepsin L. Amino acid sequencing of the N-terminus of the secreted form of MEP indicates that, during secretion, the polypeptide is cleaved between amino acids 17 and 18. We have placed the MEP cDNA in a eukaryotic expression vector and demonstrated the production of the 39 kDa polypeptide form of mouse MEP in monkey CV-1 cells.