The cysteine proteinases, cathepsins LI (SmCLl) and L2 (SmCL2) from the parasitic helminth Schistosoma mansoni were functionally expressed in Saccharomyces cerevisiae. By transforming the yeast with cDNAs encoding the complete pre-pro-enzymes, the recombinant proteinases were secreted into the culture medium from which they were purified by gel filtration and/or anion exchange chromatography. The enzymes were secreted by the yeast cells in their mature, active form and demonstrated characteristics typical of cathepsin L-like cysteine proteinases. Proteins in the purified fractions were reactive with anti-sera prepared against the recombinant enzymes which had been expressed in, and purified from extracts of Escherichia coli. Yeast-expressed SmCLl and SmCL2 displayed distinct differences in their specificities for synthetic peptide substrates; SmCLl favours substrates containing aliphatic (P3)-aliphatic (P2)-positive or polar hydrophobic (Pi) residues, whereas SmCL2 showing a preference for aliphatic (P3)-aromatic (P2)-positive (Pi). SmCLl cleaved a broader range of substrates tested and was also active over a wider pH range than SmCL2. SmCLl showed a pH optimum of 6.5 for activity against Z-Phe-Arg-NHMec, while SmCL2 was optimally active at pH 5.5 against this substrate. RT-PCR and immunoblotting studies revealed expression profiles of these proteinases also differed in the various life cycle stages of S. mansoni; both proteinases are expressed in male and female adults but at different levels, and the pro-form of SmCLl is also expressed in cercariae. Immunolocalisation experiments demonstrated that SmCLl was expressed in the gastrodermal cells lining the schistosome gut, as well as in the tegument of adult worms. Immunoblotting studies which detected the presence of SmCLl in the excretory/secretory products of adult worms suggests that this enzyme is secreted by the parasite. Moreover, recombinant yeast-expressed SmCLl had a marked preference for haemoglobin as substrate. Collectively, these results suggest that SmCLl plays some role in the degradation of host haemoglobin in the schistosome gut. SmCL2 has been localised to the reproductive organs of adult schistosomes by another laboratory and has been implicated to be involved in egg formation in female parasites. Therefore, SmCLl and SmCL2 perform different functions in schistosomes. Both these enzymes now represent novel targets at which chemotherapy and/or immunoprophylaxis may be directed.