Antioxidant enzymes in parasites play an important role in protection against the oxygen radicals by generating during aerobic metabolism, as well as in defence against host immune cell assault. Here we report the cloning and characterisation of a cDNA encoding peroxiredoxin from Ascaris suum (AsPrx). AsPrx is 776bp long and contains the nematode 22bp splice leader sequence at the 5' end and polyadenylation signal followed by poly(A) tail at the 3' end. AsPrx codes a full-length protein with a predicted molecular mass of 22. 6kDa, and possesses two cysteine residues at amino acid 49 and 168 that are conserved among Prx proteins. GenBank() analysis showed that the deduced amino acid sequence had significant similarity to parasite and mammalian Prx at the amino acid level. DNA nicking revealed that Escherichia coli-expressed recombinant AsPrx (rAsPrx) is enzymatically inhibited to form oxidative-nicking of supercoiled plasmid DNA. Two-dimensional immunoblot analysis with mouse anti-rAsPrx serum reacted two major constituent protein spots in extracts of adult female worms, suggesting that the native AsPrx might be function as a major antioxidant enzyme in Ascaris suum.