We describe one relapsed case of plasmacytoma of mandibular bone. The organs of relapse were liver and bone marrow. At relapse, monoclonal gammopathy (IgG-kappa) was observed without suppression of IgA and IgM. By immunostaining, the plasma cells of both the original mandibular bone and liver were positive for the same cytoplasmic immunoglobulin light chain kappa. The proliferative plasma cells in the bone marrow had the phenotype of CD38+, CD19+, and CD56- by flow cytometry and showed the presence of the rearranged IgH gene by Southern blotting. In addition, the zone of the Ig class of the patient's serum was not so sharply defined by zone electrophoresis. These results suggest that the characteristics of plasma cells of plasmacytoma are different from those of multiple myeloma.