Peripheral blood-localized clonal cells in patients with multiple myeloma (MM) have been reported to be insensitive to chemotherapy and it has been suggested that these cells may include the proliferative compartment in myeloma. In this study, circulating clonal CD19+ cells levels were determined in 10 patients with MM by performing patient-specific RT-PCR on single flow-sorted CD19+ cells. A variable proportion of CD+ cells being clonal were identified (0.101-6.130%, mean 1.019%). In 10/10 MM patients clonal CD19+ cells were found after high-dose treatment and peripheral blood stem cell transplantation. In conclusion, CD19+ clonal cells include a population of cells persistent after high dose chemotherapy that may be responsible for relapse.