The role of macrophages in the erythrocyte membrane oxidative damage and the pathogenesis of anemia in Babesia gibsoni-infected dogs with low parasitemia were investigated. Macrophages derived from peripheral blood monocytes (PBM) from B. gibsoni-infected dogs produced significantly higher chemiluminescent responses, indicating the release of reactive oxygen intermediates, than those from non-infected dogs when the cells were subjected to non-specific stimulation with phorbol 12-myristate 13-acetate (PMA) and opsonized zymosan (OZ), or infected dog erythrocyte membranes opsonized with infected dog serum. These results indicate that PBM of B. gibsoni-infected dogs with low parasitemia were highly activated compared to those of non-infected dogs. Furthermore, the membrane lipid peroxidation of normal dog erythrocytes incubated with PBM from B. gibsoni-infected dogs was significantly higher (p<0.05) than that of erythrocytes incubated with PBM from non-infected dogs when the PBM were stimulated with the opsonized membranes. These results suggest that the oxidative damage of erythrocytes observed in B. gibsoni-infected dogs with low parasitemia might be induced, in part, by reactive oxygen species released from the activated PBM. On the other hand, the present study also showed a significant increase (p<0.001) of IgG-bound erythrocytes in B. gibsoni-infected dogs compared with such erythrocytes in non-infected dogs. The increase of IgG-bound erythrocytes in infected dogs might reflect the increase of erythrocytes with oxidative damage induced by the infection with B. gibsoni. The results of the present study suggest that the increase of IgG-bound erythrocytes in the circulation of infected dogs induce a high degree of erythrocyte loss via immunological phagocytosis by activated macrophages, resulting in severe anemia in spite of low parasitemia.