The pattern of CSF cytological findings in the acute stage and after 10 days of antibiotic and steroid treatment was examined in 14 patients with meningoradiculitis due to an arthropode-transmitted infection with Borrelia duttoni (burgdorferi). At the acute onset of neurological symptoms, 12 patients revealed lymphocytic pleocytosis (cell count 50-500 X 10(6) cells/l) with numerous plasma cells (5-12% of the total cell count) and neutrophilic granulocytes (0.2-3.4%). 10-14 days after the onset of penicillin infusion and steroid treatment, cell count was reduced by at least 50% in 8 patients. Plasma cells then varied between 2 and 8%, whereas the neutrophilic granulocytes had disappeared. Two patients with slowly progressive neurological symptoms had a normal cell count, and inflammatory changes in CSF cytology were not observed.