We histologically and immunohistochemically studied 95 bone marrow (BM) reactive plasmacytoses. Ten biopsies from plasma cell myeloma (PCM) patients served as a control group. In addition, we studied 10 monoclonal gammopathy of undetermined significance (MGUS) cases. Histologically, plasmacytosis varied between 5% and 25% with an interstitial pattern of plasma cell (PC) distribution being characteristically displayed. Immunohistochemically, we did not find any CD56/NCAM nor cyclin D1 expression in all biopsies (95 of 95, 100%), not even a weak, doubtful one; PCs were all polyclonal and CD138 positive. On the contrary, myeloma-associated PCs showed monoclonality for kappa- or lambda- light chain and strong CD56/NCAM immunoreactivity (8 of 10, 80%); four of them were cyclin D1 positive. Osteoblasts exhibited similar CD56/NCAM expression in both groups. Our data confirm the diagnostic utility of CD56/NCAM in the phenotypic characterization of polyclonal plasma cells, suggesting an important role of this particular immunomarker in the BM trephine study of polyclonal versus neoplastic plasmacytic infiltrations.