Free radicals have previously been shown to kill the immature stages of the trematode, Schistosoma mansoni but their effect on newly excysted juvenile (NEJ) flukes of Fasciola hepatica has not been established. Using acetaldehyde and xanthine oxidase to chemically generate reactive oxygen intermediates (ROI), up to 61% of NEJ were killed but only when exposed to high levels of ROI. At low concentrations of acetaldehyde and xanthine oxidase as sources of reactive oxygen intermediates, only 6-29% of NEJ were killed compared with 70-92% of schistosomula. Incubation with lipopolysaccharide (LPS)-stimulated rat peritoneal lavage cells (PLCs) killed only 7-15% of NEJ whereas 78-87% of schistosomula were killed under the same conditions by a mechanism dependent on the production of reactive nitrogen intermediates. Relative to immature and adult parasites, NEJ expressed 2.5-20-fold lower levels of superoxide dismutase and glutathione S-transferase but no catalase activity was detected. Incubation of NEJ with inhibitors of peroxidases and glutathione metabolism increased the mean killing of NEJ by LPS-stimulated rat PLCs to 40-75%. These results demonstrate that, in comparison to schistosomula of S. mansoni, NEJ of F. hepatica are relatively resistant to killing by free radicals and this resistance could, in part, be due to the activity of oxidant scavenger enzymes of NEJ.