Two experiments on simulated postexposure treatment were carried out in dogs using human rabies immunoglobulin (RIGH) and human diploid cell vaccine for human use. In one experiment, when animals were challenged by injecting street virus into the masseter muscle and treated with a combination of RIGH and vaccine, 50% of the animals were protected from rabies. In the other trial, in which animals were challenged by injecting the virus into the femoral muscle, treatment with RIGH and vaccine protected all the animals against rabies. To our knowledge this is the highest rate of postexposure survival in animals reported to date. In addition, five out of eight (62.5%) dogs that received RIGH alone after the virus challenge were protected, while none of the animals receiving vaccine alone were protected from rabies. These trials suggest that animals can be protected from rabies by postexposure treatment. The route of exposure and timing of the administration of vaccine and hyperimmune serum would seem to be important.