Local cutaneous reactions and diffuse neurological dysfunction occurred in a patient during immunization with duck embryo rabies vaccine. Attempts to isolate an infectious agent were not successful, and it was concluded that the illness was the result of an immunologic reaction to the vaccine. The patient recovered without specific therapy. The patient's serum contained precipitating antibodies against the vaccine, and intradermal testing with the vaccine induced induration at 24 hours. These findings were interpreted to indicate that both Arthus-like reactions and delayed hypersensitivity may have to be operative in the clinical disorder.