Unconcentrated cerebrospinal fluid (CSF) and serum samples from 1114 consecutive patients were examined for presence of oligoclonal IgG bands (OB) by agarose isoelectric focusing (AIF) followed by protein transfer to nitrocellulose membrane, immunolabeling, and avidinbiotin-peroxidase staining (avidin-biotin AIF). Oligoclonal bands were demonstrated in CSF from all 58 patients with multiple sclerosis (MS), eight of 29 with aseptic nervous system infections, and 9% of 1014 with other neurological disorders (OND) considered as noninflammatory at primary clinical evaluation. Comparative examination of all specimens in another laboratory by conventional AIF after concentration of CSF revealed lower frequencies of OB in all diagnostic groups. In addition to the high sensitivity of avidinbiotin AIF, which enables detection of OB by separation of 5 microL of unconcentrated CSF even when the CSF IgG level is around the lower normal range, the procedure also has optimal specificity since IgG exclusively is detected. Avidin-biotin AIF may be the method preferred for routine examination of CSF for OB. Demonstration of OB in CSF is valuable especially in MS, where, in contrast to diagnostic aids such as evoked potentials and neuro-imaging, it establishes inflammatory type of nervous system involvements. Oligoclonal IgG bands in CSF from patients with OND reflect intrathecal immune response and should lead to investigations of infectious etiology.