Cerebrospinal fluid (CSF) and serum samples from patients with multiple sclerosis and other neurological diseases were examined by capillary isotachophoresis (ITP). The percentage and the rate of synthesis of CSF IgG which migrated slowly with ITP were calculated. CSF specimens of most patients with multiple sclerosis contained increased percentages of slowly migrating IgG (slow IgG), corresponding to IgG oligoclonal bands in the "high-alkaline" region on isoelectric focusing. The patients with multiple sclerosis were found to have increased intrathecal synthesis of slow IgG, which correlated closely with the rate of intrathecal CNS IgG synthesis calculated by Tourtellotte's formula.