An experimental biological model is proposed for purification of rickettsiae-infected cell cultures from mycoplasma contamination using intravenous mouse infection and subsequent passages on Vero cell monolayers. Mycoplasma-free rickettsial cultures were obtained in mouse brain or spleen suspension within 3 hrs or 3 days, respectively, after mouse inoculation. Ten strains of spotted fever group rickettsiae were purified from mycoplasmas by this procedure.