The B cell response against measles and herpes simplex virus (HSV) was evaluated in cerebrospinal fluid (CSF) and peripheral blood from patients with multiple sclerosis (MS) and controls by enumeration of cells secreting anti-measles and anti-HSV antibodies of IgG, IgA and IgM isotypes. We used a nitrocellulose immunospot assay which enables parallel enumeration of numbers of cells secreting total IgG, IgA and IgM. Anti-measles IgG antibody-secreting cells were present in CSF from 21 of 24 MS patients (mean 24 cells/10(4) mononuclear cells), and against HSV in CSF from seven of eight patients (mean 23/10(4) cells). No antibody-secreting cells were detectable in the patients' blood. Ten MS patients examined were negative for cells in CSF and blood producing anti-measles antibodies of IgA and IgM isotypes. Anti-measles IgG antibody secreting cells were also found in CSF from four of 18 controls, and anti-HSV IgG antibody-secreting cells in six of 13, especially in patients with subacute or chronic inflammatory nervous system diseases. Our results confirm that viral antibodies in MS are produced within CSF and that this B cell response is preferentially sequestered to this compartment. Whether this viral B cell response in MS reflects specific activation due to persistence of viral antigens or an epiphenomenon remains to be clarified.