A 53 years old female patient with plasma cell leukemia (more than 90% plasma cells in the peripheral blood, and an absolute plasma cell count reaching 147,000/mm3) is described. Electrophoresis and immunoelectrophoresis of serum and concentrated urine did not reveal any paraprotein. No surface immunoglobin has been detected on plasma cell cytoplasmic membranes. The presence of intracytoplasmic kappa light chains has been demonstrated by immunofluorescence and electron microscopy using the immunoperoxydase technique.