We have already reported that anti-erythrocyte antibody induced by Babesia gibsoni infection (aEAb) showed higher reactivity against aged and/or oxidized erythrocytes than intact one in vitro (Morita et al., 1995). To clarify the meaning of such a binding character of aEAb in vivo, changes in erythrocyte oxidation were observed in artificially infected dogs. The ratio of methemoglobin concentration against total hemoglobin concentration (metHb%) as the indicator of erythrocyte oxidation was increased by the artificial infection in the intact dogs, suggesting that aEAb played a more important role in erythrocyte destruction in infected dogs. Though parasitemia approximately three times higher was observed in splenectomized dogs than that in intact dogs, metHb% did not increase in the splenectomized dogs. This suggests that the spleen plays a very important role in erythrocyte oxidation in dogs infected with B. gibsoni.