We determined immunoglobulin G (IgG) subclass concentrations and studied their distributions in the cerebrospinal fluids of patients suffering from multiple sclerosis, other inflammatory neurological diseases, and non-inflammatory diseases of the nervous system in comparison with a control group. In addition, the four subclass concentrations were measured in serum specimens of the multiple sclerosis and control groups. These data were correlated with the extent of local IgG synthesis in the subarachnoid spaces of the patients belonging to the different groups. We found a selective elevation of the IgG1 subclass in the cerebrospinal fluids of multiple sclerosis patients, and there was only a very small overlap of the IgG1 ranges of the multiple sclerosis and control groups. No major differences were detected between the IgG subclass distributions in different courses of multiple sclerosis nor between multiple sclerosis and control sera. The group with non-inflammatory diseases showed a uniform elevation of all four subclasses and a greater overlap with the normal range. This latter feature was combined with an elevated IgG1 concentration in the group with other inflammatory diseases. It is concluded that locally synthesized IgG in the cerebrospinal fluids of multiple sclerosis patients consists mainly of IgG1.