IgM and IgG neutralizing antibody responses were estimated in the sera of rabbits and mice immunized with different doses of Semple, duck embryo and cell culture rabies vaccines. IgM synthesis was prolonged in animals immunized with multiple doses of either Semple or duck embryo vaccines but not those immunized with cell culture vaccine. Mice were passively protected by IgG antibody against subsequent challenge but not by IgM of equivalent neutralizing titre. Mice challenged when the antibody response was solely IgM were not protected if the transition to IgG synthesis was prevented by cyclophosphamide treatment.