Immunological differentiation between neuroborreliosis and multiple sclerosis. Neuroborreliosis, a tick-borne spirochaetosis of the central nervous system, is diagnosed by the presence of intrathecally synthesized Borrelia burgdorferi-specific antibodies. Multiple sclerosis and neuroborreliosis can show similarities in clinical symptoms as well as lymphocytic cell reactions and oligoclonal bands in the isoelectric focusing of cerebrospinal fluid. To differentiate between multiple sclerosis and neuroborreliosis we tested intrathecally synthesized IgM and virus antibodies. The IgM indices were higher for most of the neuroborreliosis patients studied than for those with multiple sclerosis, and cell counts were also significantly higher in the acute stage of the disease. In 84% of multiple sclerosis patients we were able to demonstrate intrathecal antibody production against measles, rubella or mumps virus. Neuroborreliosis patients had no intrathecal virus antibody synthesis. The specification of oligoclonal bands resulting from isoelectric focusing of cerebrospinal fluid with an ELISA for B. burgdorferi can further substantiate the diagnosis of neuroborreliosis or help to rule it out in multiple sclerosis patients with positive borrelia-specific serology. 