Cerebrospinal fluid (CSF) involvement in myeloma is rarely seen. Recently we experienced a case with this lesion. A 70-year-old man developed consciousness level disorder during the course of bronchopneumonia. Neurological examination revealed stuporous consciousness, neck stiffness and Kernig's sign. Immunoelectrophoresis showed monoclonal IgG in serum. CSF which was obtained through lumbar puncture was clear and its pressure was 155 mm H2O. It contained 207 white cells/3 mm3; glucose, 54 mg/dl; and protein, 33 mg/dl. The differential count of the CSF was (in %) monocytes, 48.0; plasma cells, 25.5; neutrocytes, 15.5; and lymphocytes, 11.0. Cytoplasm and nucleus of the plasma cells were in various sizes. Some irregular multiple nuclei, flaming cells and grape cells were also observed in them. The cytoplasm of the plasma cells fluoresced with antisera against lambda chains IgG. The value of immunofluorescent technique in identifying plasma cells in the CSF is emphasized.