rFhGST-S1 was produced as an active protein, displaying significant enzymic activity towards the model GST substrate 1chloro-2,4-dinitrobenzene (CDNB) and a range of substrates commonly used to characterise GSTs (Table 1) . F. hepatica GST is very similar in terms of its enzymatic profile to the GST of S. Sigma Class Glutathione Transferase of F. hepatica www.plosntds.org japonicum currently undergoing clinical vaccine trials. FhGST-S1 also displays higher glutathione-dependent lipid peroxidase activity compared to both Sm28GST and Sj26GST [47] . Interestingly, ligand inhibition studies on rFhGST-S1 showed the enzymic activity of rFhGST-S1 with CDNB was inhibited by the major pro-active form of the main liver fluke drug Triclabendazole. The sulphoxide derivative (TCBZ SO) gave an IC50 (50% enzyme inhibition) of 5765 mM (5 replicates). Bile acids, potentially natural ligands for liver fluke tegumental associated proteins in the host bile environment, were also assessed for activity inhibition. The rFhGST-S1 interacted with all three bile acids tested using five replicate assays: Cholic acid (IC50 302673 mM); Deoxycholic acid (IC50 223621 mM) and Chenodeoxycholic acid (IC50 6469 mM) .