In this article, we review neoplastic contamination in the peripheral blood (PB) of patients with multiple myeloma (MM) upon stem cell mobilization. We first evaluated PB samples from pretreated MM patients following administration of high-dose cyclophosphamide (Cy, 7 g/m2 or 4 g/m2) and granulocyte colony-stimulating factor (G-CSF) for the presence of myeloma cells as well as hematopoietic progenitors. Plasma cells containing intracytoplasmic immunoglobulin (cIg) were counted by immunofluorescence microscopy after incubation with appropriate antisera against light and heavy chain Ig. Flow cytometry studies were performed to determine the presence of malignant B lineage elements, using monoclonal antibodies against the CD19 antigen and the monotypic light chain. Prior to PBSC mobilization, circulating plasma cells were detected in all MM patients at 0.1%-1.8% of the mononuclear cell (MNC) fraction (mean value 0.7 +/- 0.4% SD). In these patients, a higher absolute number of PB neoplastic cells was detected after administration of chemotherapy and G-CSF. Kinetic analysis showed a pattern of tumor cell mobilization similar to that of normal hematopoietic progenitors, with the peak coinciding with the optimal period for the collection of PBSC. The absolute number of plasma cells showed a 10-50-fold increase over the baseline value. Apheresis products contained 0.7 +/- 0.2% SD myeloma cells (range 0.2%-2.7%), which demonstrated the capacity of plasma cells to proliferate, differentiate, and mature in response to c-kit ligand (SCF), IL-3, IL-6, and a combination of IL-3 and IL-6. Subsequently, in an attempt to reduce tumor cell contamination prior to autologous transplantation, circulating hematopoietic CD34+ cells were highly enriched by avidin-biotin immunoabsorption, cryopreserved, and used to reconstitute bone marrow (BM) function after myeloablative therapy in 13 patients. The median purity of the enriched CD34+ cell population was 89.5% (range 51%-94%), with a 75-fold enrichment compared with the pretreatment samples. The median overall recovery of CD34+ cells and CFU-GM was 58% (range 33%-95%) and 45% (range 7%-100%), respectively. Positive selection of CD34+ cells resulted in 2.5-3 log depletion of plasma cells and CD 19+ B lineage cells as determined by immunofluorescence studies, although DNA analysis of the CDR III region of the IgH gene demonstrated the persistence of minimal residual disease (MRD) in 5 of 6 patient samples studied. Myeloma patients were reinfused with enriched CD34+ cells after myeloablative therapy consisting of total body irradiation (TBI, 1000 cGy) and high-dose melphalan (140 mg/m2) or melphalan (200 mg/m2) alone. They received a median of 5 x 10(6) CD34+ cells/kg and showed a rapid reconstitution of hematopoiesis. The median time to 0.5 x 10(9) neutrophils, 20 x 10(9) and 50 x 10(9) platelets/L of PB was 10, 11, and 12 days, respectively. These results, as well as other clinically significant parameters, did not significantly differ from those of patients (n = 13) receiving unmanipulated PBSC following the same pretransplant conditioning regimen. Our data demonstrate the concomitant mobilization of tumor cells and hematopoietic progenitors in the PB of MM patients. Positive selection of CD34+ cells reduces the contamination of myeloma cells from the apheresis products up to 3 log and provides a cell suspension capable of restoring normal hematopoiesis following a TBI-containing conditioning regimen.