Antibodies to human spinal-cord proteins in sera from patients with motor neuron disease (MND) and other neurological diseases were detected by the immunoblotting method. IgG in serum from a patient with amyotrophic lateral sclerosis reacted with insoluble proteins in the crude nuclear subfraction but the IgG in serum from other patients reacted with soluble proteins. The molecular weights of the antigens on the blots differed among the cases. The significance of the antibodies to neural tissue in sera from MND remains unknown. The etiology of MND is thought to be heterogenous and some types of MND may be closely related to humoral immunity.