Bone marrow samples from 55 patients with multiple myeloma (MM) and 23 patients with monoclonal gammopathy of undertermined significance (MGUS) were evaluated with a broad panel of monoclonal antibodies. Plasma cells from 78% (43/55) of patients with MM strongly expressed the natural killer cell antigen CD56 (NKH-1, Leu-19). Of the 23 patients with MGUS, none showed strong CD56 reactivity, although three had weak reactivity in less than 20% of plasma cells. Myeloma cells expressing CD56 did not coexpress the CD57 or CD16 antigens. Patients with CD56-positive plasma cells had both indolent and aggressive disease. However, the 12 CD56-negative patients had predominantly aggressive disease with an unexpected preponderance of kappa Bence Jones only myeloma (5/10[50%] evaluable patients). Polyclonal plasma cells from non-neoplastic tissue sites (normal bone marrows, lymph nodes, tonsillar biopsies, and gut-mucosa biopsies) showed a near absence of CD56. We conclude that isolated, strong CD56 expression is common in MM, but not in MGUS or reactive plasma cells. The potential biologic importance of CD56 positivity in myeloma is reviewed.