Autoantibodies to neurofilaments were found by the immunofluorescence technique in serum of patients with postencephalitic (von Economo's) and idiopathic Parkinson's disease in the same proportion as in age-matched neurological and non-neurological controls. In addition, similar neurofibrillary staining was detected in age groups of 29 years and younger, but rarely in the first year of life. Persons over 70, with or without disease, showed a prevalence of antibodies significantly higher than in persons under 70. Serum from 1 case of Alzheimer's disease out of 4 tested, was positive for neurofilament antibodies; serum from the only case of Creutzfeldt-Jakob disease tested was negative. A total of 298 serum specimens, each from a different person, was tested. The use of cryostat-frozen longitudinal sections of normal rat spinal cord as a substrate has been confirmed to be an effective, reproducible and simple procedure for the detection of antineurofilament antibodies in human sera by indirect immunofluorescence.