Effects of ageing treatment of erythrocytes to binding activity of the anti-erythrocyte antibody(s) in the serum of dogs infected artificially with Babesia gibsoni were examined by enzyme-linked immunosorbent assay (ELISA) using fixed cell antigen. When erythrocytes made senescent by two artificial ageing treatments, one of which was storing erythrocytes at 37 degrees C in medium for some days and the other was oxidation of erythrocytes with phenylhydrazine, reactivity of anti-erythrocyte antibody(s) in the infected serum to both types of treated erythrocytes was increased. These results suggest that such antibody(s) could play a role as one of the causes of anemia in canine babesiosis.