Non-denaturing polyacrylamide gels were used to analyse superoxide dismutase (SOD), catalase, peroxidase, NADH oxidase and NADH peroxidase in the microaerophilic protozoan parasite Giardia duodenalis. A cytosolic H2O-producing NADH oxidase and membrane-associated NADH peroxidase were readily detected from G. duodenalis. In all Giardia strains investigated the NADH oxidase was present in high levels (1.2-2 U (mg protein)-1). Using the same technique, NADH oxidase activity was also detected in the microaerophilic protozoan parasites Tritrichomonas foetus, Trichomonas vaginalis and Entamoeba histolytica and in the bacterium Escherichia coli. The conventional enzymes of oxidative stress management (superoxide dismutase, catalase and peroxidase) were not detected in particulate or cytosolic extracts from recent and established strains of Giardia assayed in situ. Spectrophotometric assays also yielded negative results. The same methodology readily detected one or more of these enzyme activities in T. foetus, T. vaginalis and E. coli. Superoxide dismutase activity was not detected in lines of Giardia resistant to high levels of metronidazole or furazolidone. Furthermore, the agents 1,10 phenanthroline, diamide, MnCl2 and KNO3, which induce SOD in anaerobically cultured E. coli, did not induce SOD in Giardia. 1,10 phenanthroline has also been shown to induce iron-containing (Fe-) SOD in Entamoeba. Neither peroxidase nor catalase activities were detected in a peroxide-resistant line of Giardia. Viable trophozoites from parent lines were able to decompose H2O2 at a significant rate. It appears that the conventional SOD, catalase and peroxidase utilised in aerobic metabolism have been substituted in Giardia by NADH oxidase and NADH peroxidase, similar to anaerobic bacteria. The O2-scavenging NADH oxidase explains the previously observed futile 'respiration' in Giardia.