Amyotrophic lateral sclerosis may be an autoimmune disease. In this paper IgG subclasses levels in the CSF and sera and their intrathecal synthesis were studied. IgG subclasses levels were determined by ELISA method using monoclonal antibodies against human IgG subclasses, secondary biotinylated antibody and avidin-biotin-peroxidase complex. There was statistically significant elevation of IgG1 and IgG3 subclasses in the CSF of ALS patients. In sera of patients with ALS, IgG2 level was diminished, but there was no statistical difference in other IgG subclasses. IgG1 and IgG3 indices were elevated in patients with ALS, detecting synthesis of these subclasses in the CNS. General IgG index value did not differ from the control value. The results support the concept that autoimmune mechanisms may play a role in the pathogenesis of ALS.