Mice which have recovered from Babesia microti infection lose their parasitaemia as soon as three weeks after recovery and become solidly immune. This sterile immunity is not affected by splenectomy and may last for the life of the mouse. Mice which have recovered from B. hylomysci infection continue to harbour parasites at a subpatent level and spontaneous relapses were common after recovery. Hence the type of immunity which developed against this parasite is premunition and may last for life. Such immunity was not absolute and considerable parasitaemias developed after challenge. Cross-protection occurred between the two species of parasite.