A leptomeningeal cell line (clonal but not immortal) was isolated from lumbar-punctured cerebrospinal fluid in a patient with definite multiple sclerosis (MS). This cell line, named LM7, was characterized by immunocytochemical and ultrastructural analyses and was found to produce specific viral reverse transcriptase activity, whereas electron microscopy revealed the presence of viral particles. This patient had no antibodies against human T-leukaemia virus type 1 (HTLV-I) or human immunodeficiency viruses types 1 and 2 (HIV-1, HIV-2). Moreover, monoclonal antibodies against HTLV-I and HIV-1 failed to recognize epitopes in induced LM7 cells. The possible relationship between MS and the virus present in LM7 cells is discussed.