CSF and serum from 700 patients were analysed by isoelectric focusing (IEF). Oligoclonal bands in the high alkaline range were detected in 156 cases. Ninety-five patients were diagnosed as having multiple sclerosis (MS), 38 various CNS inflammatory disorders, 18 vascular, neoplastic or degenerative and 5 patients other neurological diseases. IEF was positive in 98% of clinically definite and 85% of clinically probable MS cases and was found to be more sensitive as compared to intrathecal IgG production and evoked potentials. Neither the presence of oligoclonal IgG nor the banding distribution pattern is specific for any disease. Further characterization may be obtained by immunofixation if the antigen is known.