Sigma class GSTs from both parasites and mammals have been known to exhibit prostaglandin synthase activity. To this end, the Sigma GST from F. hepatica shares a high sequence identity with recognised Sigma class GSTs with prostaglandin synthase activity, including rOvGST-1 from the filarial parasite, Onchocerca volvulus. Using a coupled assay with COX-1 we have shown that rFhGST-S1 is capable of synthesizing both PGD 2 and PGE 2 , with PGD 2 being the predominant prostanoid. Parasite-derived eicosanoids, including prostaglandins, are known to be important in the establishment of parasitic infection and the survival and proliferation within the host. Therefore, eicosanoids produced by parasitic helminths may play a role in pathophysiological changes during helminth infections. For example, chronic fasciolosis is associated with fever and changes in liver biochemistry, both of which could be associated with parasite-derived eicosanoids thromboxane B2 (TXB2), PGI 2 , PGE 2 and leukotriene B4 (LTB 4 ), detected in the ES products and homogenates of adult F. hepatica worms [56] . In addition, the migration of host epidermal Langerhans cells, which play a key part in immune defence mechanisms, has been shown to be inhibited by parasite-derived PGD 2 in the Schistosoma mansonimouse model of human infection, thus allowing schistosomes to manipulate the host immune system [57] . Earlier studies have revealed the presence of eicosanoids produced by S. mansoni cercariae which could also play a role in establishment of infections through loss of the cercarial tail following penetration of the skin [58] . It therefore seems likely that prostaglandins synthesised via FhGST-S1 will have a role in establishing the infection within the host.