The antineurofilament antibodies found in the serum of a chimpanzee with experimental Creutzfeldt-Jakob disease reacted specifically with the 200,000-dalton polypeptide of the purified neurofilament triplet. They also reacted strongly with thoroughly characterized neurofilament swellings of proximal axons of spinal cord motoneurons from beta,beta' iminodipropionitrile (IDPN)-intoxicated rats.