The T lymphocytes in blood and CSF of 14 patients with acute polyradiculoneuritis were studied using the OKT series of monoclonal antibodies. Results were compared with findings in two control groups. Three patients with cytomegalovirus infection showed elevation of circulating OKT 8. A significant increase in the mean ratio OKT 4/OKT 8 was noted in all other cases, corresponding in 4 of the 11 patients to a reduction in number of OKT 8 lymphocytes. The CSF was normal in 3 of the 14 cases.