Immunofluorescent examination of fresh and in vitro cultured CSF cells was used for detection of viral central nervous infections in two cases of mumps meningoencephalitis and in two with diffuse sclerosing central nervous system disease. In contrast to the weak fluorescence found in sedimented CSF cells, cultured CSF cells showed a bright specific fluorescence. The combination of CSF cell culture with direct or indirect immunofluorescence of fresh cells may provide more cells for the immunofluorescent technic and also increase the virus yield.