Aerobic organisms depend on superoxide dismutase to suppress the formation of dangerous species of activated oxygen. Intraerythrocytic stages of the malaria parasite exist within a highly aerobic environment and cause the generation of increased amounts of activated oxygen. Plasmodium berghei in mice was found to derive a substantial amount of superoxide dismutase activity from the host cell cytoplasm. Plasmodia isolated from mouse red cells contained mouse superoxide dismutase, whereas rat-derived parasites contained the rat enzyme. This is believed to be the first example of the acquisition of a host cell enzyme by an intracellular parasite.