A cDNA encoding Schistosoma mansoni cathepsin L was isolated from a cDNA library and sequenced. Alignment of the proposed amino-acid sequence with known members of cathepsin L shows highest homologies with sequences from mouse and rat. An expression plasmid was constructed in Escherichia coli to produce recombinant schistosome cathepsin L with an extension of six histidines at its N terminus. Using antibodies raised against the purified fusion protein, two polypeptide bands with approx. molecular masses of 38 and 31 kDa were identified in a schistosome extract. By use of specific radioiodinated inhibitors, a radioactively labeled protein could be detected at 31 kDa, suggesting that this is the active mature enzyme. The larger protein of 38 kDa did not react with the inhibitor, indicating that it represents the inactive precursor molecule. Immunohistological experiments revealed that the proteinase is localized in structures associated with the reproductive system of females and with the subtegumental region of the gynecophoric canal of males. However, Northern blot hybridization demonstrates that more transcripts are present in female parasites than in males. Genomic Southern blotting suggests that schistosome cathepsin L is expressed from a single-copy gene.