Fasciola hepatica, a parasitic trematode, is the causative agent of liver fluke disease. It has been shown previously, that both the migratory and adult worm stage of the parasite secrete multiple cysteine proteinases when they are cultured overnight (Dalton & Heffernan, 1989). In this study, one of these proteinases has been purified by standard chromatographic techniques. The purified enzyme was characterised as a cathepsin L-like proteinase using synthetic substrates, inhibition studies, N-terminal sequencing and immunolocalisation studies. This is the first cathepsin L-like proteinase to be identified in a parasitic trematode. This cathepsin L-like proteinase is capable of cleaving immunoglobulin molecules, and is able to protect newly excysted juveniles from destruction by immune-effector cells when it is included in an eosinophil adherence assay. Antibodies to the purified proteinase are able to neutralise its proteolytic activity in vitro. A partial gene fragment encoding the cathepsin L-like proteinase has been obtained using PCR and subcloning techniques. The cathepsin L-like proteinase is present in all stages of F. hepatica and, hence, is considered an ideal target molecule at which to design a vaccine and/or drug, for use in the control of this agriculturally important parasitic disease.