Arachidonic acid was co-oxidized by xanthine oxidase. Both superoxide radical and hydrogen peroxide were required for oxidation, as shown by essentially complete inhibition caused by superoxide dismutase or by catalase. Pure arachidonate, free of lipid hydroperoxides, was susceptible to this co-oxidation, and the presence of lipid hydroperoxides did not accelerate the process. The role of trace metals was indicated by the stimulatory effect of EDTA-Fe and by the inhibitory effect of diethylenetriamine pentaacetate. Initiation of arachidonate co-oxidation was due to a potent oxidant generated by the interaction of H2O2 and O2- in the presence of Fe, rather than to either O2- or H2O2 per se. Hence, mannitol, a scavenger of OH ., but not of O2- or H2O2, also inhibited oxidation. Arachidonic acid autoxidation, a much slower process than xanthine oxidase co-oxidation, was barely detectable on the time scale of these observations. Unlike the co-oxidation, autoxidation was autocatalytic and therefore accelerated by hydroperoxide products. Marked quantitative differences in the distribution of isomeric hydroperoxide products of enzymic co-oxidation, as compared to the autoxidation, were noted and their significance was discussed.