A 30-kDa protease, purified previously from Fasciola hepatica, was sequenced and the first 15 N-terminal residues were found to be 100% homologous to a region in the protein Fcp1c, which was cloned and expressed from F. hepatica. This terminal region was also 53 and 54% identical to two other cathepsin L-like proteases isolated from the same source. The 30-kDa protease demonstrated a specificity different from human cathepsin L when assayed with novel peptidyl enediones of the type Z-Phe-Ala-CH=CH2-CO2R (where R = Me/Et/Bu-t). The ethyl ester peptide was a more efficient inhibitor of the protease than the corresponding methyl ester. This is in contrast to bovine cathepsin B and human cathepsin L where both are more readily inhibited by the methyl, rather than the ethyl ester peptide. These differences in the inhibition of the novel parasite protease may allow it to be exploited as a chemotherapeutic target.