Helicobacter pylori sonicate was shown to oxidize ascorbic acid. Ascorbic acid oxidation was determined by chromatography combined with electrochemical detection. Water soluble ascorbic acid oxidase activity was rather independent of pH with a pH optimum around 2. By gel filtration the oxidizing activity co-eluted with an absorbency peak at 408 nm. The relative molecular mass (Mr) was approximately 14,000. It is suggested that this oxidating activity was caused by a cytochrome c-like molecule. Ascorbic acid oxidating activity could also be extracted from bacterial membranes by detergents. Gel filtration showed several forms, the major one with a Mr = 19,000. pH optimum was 6-7. Other oxidase-positive bacterial strains like Campylobacter coli, Enterobacter cloacae and Pseudomonas aeruginosa could degrade ascorbic acid. Since ascorbic acid oxidation by Helicobacter pylori whole bacterial lysates has a pH optimum in the acidic range corresponding to pH in gastric fluid, the activity of the cytochrome c-like water soluble oxidant of Helicobacter pylori seems to be primarily important for the destruction of ascorbic acid in the gastric juice of infected patients.