Intrathecal B cell function in healthy subjects has been poorly elucidated. Although there are measurable quantities of immunoglobulins (Ig) in the cerebrospinal fluid (CSF) of 'normal' individuals, it is not clear whether this reflects transudation from serum or is due to some production within the central nervous system. We have therefore isolated cells from CSF to assess the frequency of Ig-secreting cells, utilizing a nitrocellulose immunospot assay for enumeration of the IgG-, IgA- and IgM-producing cells per 10(4) mononuclear cells (MNC) isolated from CSF and blood. Contrary to previous belief, the CSF obtained from 22 of 23 'normal' subjects (95%) with muscular tension headache but no evidence of organic neurological disease contained 2-50 (mean 20) IgG-secreting cells per 10(4) MNC. The corresponding peripheral blood specimens contained 0-6 (mean 3) IgG-secreting cells per 10(4) MNC. The proportion of IgG-secreting cells among MNC is thus about 7-fold higher in CSF than in blood of healthy individuals. IgA- and IgM-producing cells were also found in normal CSF, but less frequently than cells secreting IgG and at proportions similar to those in peripheral blood. We suggest that there is continuous production of Ig of different isotypes in CSF, even in subjects without any signs of neurological disease.