Multiple myeloma(MM) is characterized by the clonal expansion of malignant plasma cells. In xenograft models, CD19-CD38++ MM plasma cells engrafted and rapidly propagated MM, indicating that MM plasma cells, which are terminally differentiated cells, include MM-initiating cells. Bone marrow niche for MM-initiating cells are now being investigated extensively. MM patients harbor phenotypic CD19+ B cells expressing the immunoglobulin gene sequence and the idiotype unique to the individual myeloma clone. CD19+ clonotypic B cells include "pre-myeloma stem cells", and additional oncogenic hits are likely to be needed before developing myeloma disease from them. Prospective identification of CD19+ "pre-myeloma stem cells" is important for diagnosing pre-myeloma status and for developing the methods for the prevention of multiple myeloma.