A nitrocellulose immunospot assay that allows the counting of cells secreting IgG, IgA, or IgM antibodies to Borrelia burgdorferi was used to compare B cell response to B burgdorferi at the cellular level in cerebrospinal fluid (CSF) and blood from patients with neuroborreliosis with that in patients with aseptic meningoencephalitis (AM) or non-inflammatory neurological diseases. 13 of the 14 patients with untreated neuroborreliosis had CSF cells secreting IgG antibodies to B burgdorferi (mean 17 cells per 10(4) CSF cells), whereas 8 of 12 patients examined had cells secreting IgA antibodies (mean 6 cells) and 10 of 12 had cells secreting IgM antibodies (mean 6 cells) per 10(4) CSF cells. IgG antibody producing cells predominated except in 2 patients with mainly or only IgM secreting cells. Cells secreting antibodies to B burgdorferi were rarely found in the blood and then at very low numbers, which reflects preferential compartmentalisation of the specific B cell response to the CSF. The cells were not detectable in CSF or blood from the two control groups. Evaluation of humoral immunity at the cellular level is a novel approach to the detection and localisation of immune events in neuroinflammatory disorders.