Four murine anti-idiotypic (a-Id) hybridoma antibodies were produced against immunoglobulins (Ig) present in the cerebrospinal fluid (CSF) obtained from an MS patient 2 mo after the onset of disease. The four a-Id antibodies were shown to delineate idiotopes present on three distinct Ig subpopulations designated ID-19, ID-40, and ID-97. All three Ig subpopulations were produced in part by intrathecally localized B cells, together making up approximately 5% of the total CSF-Ig 2 mo after the onset of disease. Longitudinal analysis of the concentration of these Ig subpopulations in CSF showed that two subpopulations, ID-40 and ID-97, exhibited a regular relation to the clinical course of the disease, i.e., were decreased (ID-40) or increased (ID-97) in the first CSF sample obtained after two consecutive exacerbations. Screening of sera from 52 optic neuritis patients and 51 heterologous MS patients revealed that one MS patient's serum contained an Ig subpopulation that was idiotypically cross-reactive with ID-97. So far, screening of these Ig subpopulations for reaction with several viruses (measles, parainfluenza type 1, influenza type A, cytomegalovirus, herpes simplex virus, rubella virus, poliovirus, murine encephalomyelitis viruses, and reovirus) and myelin basic protein has failed to reveal their antigen specificities.