It is known that rubella virus is isolated from the cerebrospinal fluid (CSF) of young infants with congenital rubella. (1, 3, 5, 7) On the contrary it is difficult to isolate the virus from the CSF of patients with acquired rubella encephalitis. We examined the cells in the CSF from a neonate with congenital rubella syndrome, in comparison with the cells of acquired rubella, by the immunofluorescence method.A case, M.T., male. His mother had rubelliform rash on the 38th day of gestation. The baby was born 11 days later than the expected date, with a weight of 2,100 g, microcephaly, cataracts, suggestive of congenital heart disease, left inguinal hernia and probably deafness which became distinct later. Titers of hemagglutination inhibition (HI) antibody in the baby's serum and in his mother's were 1 :256 and 1:512, respectively. Specific IgM antibody was contained only in the former. On the 23rd day of age, spinal tap was done, because of suspicious signs of intracranial involvement, and a small amount of CSF was obtained, in which 67 lymphocytes and 3 polymorphonuclear cells were contained per cubic millimeter.As controls were examined CSF cells of 15 cases with central nervous system (CNS) complication of acquired rubella and of 16 cases with CNS diseases without rubella infection.The CSF was centrifuged at 1,500 rpm for 15 min. The sediment was washed once with 10 ml of PBS, pH 7.2, then smeared on cover-slips, dried in room temperature and fixed in cold aceton at -20 C for 15 min. An indirect immunofluorescent technique was applied with a hyperimmune rabbit serum against rubella virus M33 (supplied by Dr. J. Nonaka, the Chemo-Sero-Therapeutic Research Institute) and a goat anti-rabbit gamma-globulin conjugated with fluorescein isothiocyanate (FITC) (Behringwerke). The rabbit serum whose HI antibody titer was 1 :10,240, was diluted to 1 :500 after absorption with bovine liver powder (NBC). A blocking test to testify its specificity was done with human convalescent serum with an HI antibody titer of 1:256, including IgG and IgM antibodies. Specificity of immunofluorescence was repeatedly examined also with rubella virus-infected BHK-21 and Vero cells (6).The specific immunofluorescence observed was diffuse in the cytoplasm and granular in the nucleus of lymphocytes derived from the CSF of the neonate with congenital rubella. Only cytoplasmic fluorescence was seen in lymphocytes of the 731