This study demonstrates that the multiple myeloma cell (MMC) in its plasma cell form is morphologically indistinguishable from human osteoclast-like cells that form in culture when peripheral blood mononuclear cells (PBMCs) are plated at high density in serum containing medium. MM has been described as a disease of B-cell lineage, monoclonal immunoglobulin (Ig) producing cells with unique properties: MM precursor cells lodge in bone, where they proliferate and differentiate into plasma cell tumors. Then, by some mechanism, presumably involving cytokines, these cells mediate an increase in neighboring osteoclast numbers and activity, leading to excessive bone erosion and hypercalcemia. Three days after plating PBMCs, tartrate resistant acid phosphatase-(TRAP-) blasts as well as TRAPϩ cells, each with an eccentric nucleus, appear in culture. By day 10, TRAPϩ, vitronectinϩ (VRϩ) cells, appear to be morphologically indistinguishable from multiple myeloma plasma cells (MMPCs) on cytocentrifuge preparations. These cells are CD19-and CD38ϩϩ, as are MMCs reported by others. Other surface markers are also shared. Furthermore, Ig mRNA is demonstrated in the cytoplasm of cells at 8 days by in situ hybridization with the IgG FcA3 sequence. This novel finding is not unusual, in light of reports, demonstrating non-B-lineage Ig-producing cells. Thus, this study raises some serious questions about the true nature of MMCs.