Cerebrospinal fluid immune complexes in patients exposed to Borrelia burgdorferi: detection of Borrelia-specific and -nonspecific complexes. We analyzed cerebrospinal fluid (CSF) from 32 patients with neurological symptoms and evidence of Borrelia burgdorferi infection (29 were seropositive as determined by enzyme-linked immunosorbent assay, 2 were cell-mediated immune positive, and 1 had been seropositive as shown by enzyme-linked immunosorbent assay 9 months previously). CSF immune complexes were found in 22 (69%) of 32 patients; in 18, there was sufficient sample to isolate immune complexes. By enzyme-linked immunosorbent assay, isolated immune complexes from 10 of these 18 patients contained antibody specific for B. burgdorferi antigens. The isotypes were IgG (n = 8), IgM (n = 3), and IgA (n = 2). By immunoblot, these antibodies were directed against B. burgdorferi 41-kDa antigen and occasionally against the 33- and 17-kDa antigens. Anti-B. burgdorferi IgM was present in patients with acute neurological symptoms, was predominantly complexed rather than free, and decreased with clinical recovery in the one serial study. Three patients were nonreactive for free CSF antibodies, but had complexed antibodies to the organism. The preliminary finding of specific B. burgdorferi components in immune complexes in CSF suggests an active process triggered by the organism, even in the absence of other CSF abnormalities. 