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Immunity of rats and mice following injection with 60Co irradiated Babesia rodhaini infected red cells

Babesia rodhaini infected mouse and rat red cells were non-infective after receiving 40 or 60 krad irradiation from a 60Co source. Rats inoculated with irradiated infected red cells were subsequently found to show strong resistance to homologous challenge. Mice, which are more susceptible to B. rodhaini, showed less resistance to challenge, following immunization with irradiated B. rodhaini infected red cells. Splenectomized rats were immunized with irradiated infected blood. There was a delay compared with the controls in the onset of acute fatal parasitaemias in these rats. A population of parasites was isolated by taking and mixing blood from each of the splenectomized rats during the patent parasitaemia following challenge. This population appeared to be an antigenic variant of the original immunizing population. I thank Mrs J. Page for excellent technical assistance and Dr K. N. Brown for criticizing the manuscript.