Oligoclonal IgG was present in the cerebrospinal fluid (CSF) of each of three patients with neurosyphilis studied. Conventional serological tests disclosed reduced serum/CSF ratios of antibodies to Treponema pallidum (TP) in each patient, consistent with intrathecal production of treponemal antibodies. Antibody analyses of electrofocused specimens by an immunofixation technique disclosed intrathecal production of oligoclonal TP antibodies in all patients. Treponemal antibody light chains showed a close correlation with the light chains of the oligoclonal CSF IgG. Absorption of the CSF with TP caused removal of the oligoclonal IgG. The results provide strong evidence that the oligoclonal CSF IgG in neurosyphilis represents TP antibodies, reflecting a specific immune response in the central nervous system to the infectious agent.