The intrathecal synthesis of borrelial-specific IgM- and IgG-antibodies was studied in 67 patients with neuroborreliosis and in 14 patients with neurosyphilis (controls). Antibody concentrations in serum and in the cerebrospinal fluid were determined by an enzyme immunoassay (EIA) using, as antigens, a sonicate of Borrelia burgdorferi, the recombinant 14 kDa flagellin fragment, the outer surface protein C (22 kDa), and the high molecular mass protein p83 (83 kDa). In the sonicate EIA, IgG- and/or IgM-antibodies to Borrelia burgdorferi in serum were detected in all patients with neuroborreliosis and in 71% of patients with neurosyphilis. Intrathecal synthesis of borrelial-specific IgG- and/or IgM-antibodies was demonstrated in 82% of patients with neuroborreliosis and in 71% of patients with neurosyphilis. Immunoglobulin G- and/or IgM-antibodies in serum against any of the recombinant antigens were detected in 92% of patients with neuroborreliosis and in none of those with neurosyphilis. Intrathecal synthesis of IgG- and/or IgM-antibodies to individual recombinant antigens was demonstrated in 67% of patients with neuroborreliosis and in none of those with neurosyphilis. The sensitivity of the recombinant antigens in serum was almost equal to that of the sonicate EIA, whereas the recombinant antigens were clearly less sensitive in the estimation of the intrathecal specific immune response. It was concluded that in suspected cases of neuroborreliosis, the estimation of high specific antibodies in the recombinant EIA will be helpful in confirming the diagnosis.