Smears of cells of cerebrospinal fluid obtained from patients with Japanese B encephalitis were examined by means of the fluorescent antibody method. Identification of cell types were maid in preparations stained with Giemsa solution after removal of fluorescein-conjugated antibodies. Specific fluorescence was observed in a part of phagocytes, which were the predominant cell type in cerebrospinal fluid in case of Japanese B encephalitis, but no fluorescence in neutrophils. A diffuse distribution of specific fluorescence was seen in the cytoplasm, and its glanular aggregation in a part of the perinuclear region of the phagocytes. Phagocytes schowing specific fluorescence were noticed in all of patients with Japanese B encephalitis and they ranged from 15% to 48% of whole cells seen in preparations. Demonstration of the viral antigen within phagocytes in cerebrospinal fluid by the fluorescent antibody method may contribute to the early diagnosis of Japanese B encephalitis.