Clearance of trypanosomes from the blood of infected Cape buffalo was associated with the development of two responses: (i) complement-dependent and clone-specific lytic activity and (ii) complement-independent trypanocidal activity that was not restricted by trypanosome clone or species. This latter activity was mediated by H₂O₂ and required the presence of xanthine oxidase in serum but not the addition of purine substrates. Expression of the xanthine oxidase-dependent trypanocidal activity in Cape buffalo serum was coincident with, and required, a decline in its H₂O₂ catabolic activity. The H₂O₂ catabolic activity of Cape buffalo serum was due solely to catalase and declined by eightfold around the time that trypanosomes were cleared from the blood, accompanied by a fivefold drop in erythrocyte-associated catalase activity. The Cape buffalo did not develop subsequent parasitemic waves. Clearance of parasitemia in similarly infected cattle was also associated with development of trypanosome clone-specific lytic activity, but not with the acquisition of H₂O₂-dependent trypanocidal activity in serum, and the cattle supported recurring parasitemia. The lack of trypanocidal activity in pre- and postinfection cattle sera was due to their low content of xanthine oxidase and sustained catalase activity. These data strongly suggest that an infection-induced serum oxidative response, the efficacy of which is amplified by a decline in blood catalase, contributes to suppression of recurring parasitemia in Cape buffalo.