Roles of immunologic factors and slow virus infections in the aetiopathogenesis of amyotrophic lateral sclerosis (ALS) have recently gained significance. For this purpose, presence of poliovirus type I, measles and herpes simplex (HSV) type I antibodies were determined in the sera of ALS patients by complement fixation technique. Significant increase in antibody titers for poliovirus type I and HSV type I were determined in sera of ALS patients in comparison with controls and patients with other neurologic disorders.