The HEP and ts2 strains of rabies virus inoculated intracerebrally into adult mice normally cause clinically inapparent infection. In the experiments described, this is converted into a lethal infection by immunosuppression with cyclophosphamide, which also prevented induction of immunity with vaccine. Lethal infection of HEP-inoculated mice was also observed in mice treated with antithymocytic serum, and in athymic (BALB/c-nu) nude mice.