Multiple myeloma is basically an incurable cancer. Most patients respond initially to chemotherapy with reduction in bone marrow (BM) plasma cells and monoclonal Ig levels, but the disease nearly always recurs and becomes refractory to therapy. The objective of this study was to characterize the expression of the multidrug transport pump, P- glycoprotein 170 (P-gp), in myeloma. The great majority of B cells from peripheral blood mononuclear cells (PBMCs) in myeloma express P-gp, detected by the monoclonal antibody MRK-16. P-gp+ blood B cells exhibit extensive DNA hyperdiploidy, suggesting replicative abnormality characteristic of malignant growth. We speculate these represent a stem cell population in myeloma. The proportion of B cells expressing P-gp was comparable among untreated myeloma patients and those treated with chemotherapy, biologic response modifiers, or off treatment. Among BM cells, P-gp was absent or low in untreated myeloma patients but was expressed at high levels on BM cells from patients previously treated with chemotherapy. For untreated patients the majority of B/plasma cells expressing P-gp are located in PBMCs, not the BM cells. Flow cytometric analysis of rhodamine 123 dye efflux indicated a functional P-gp that was efficiently blocked by verapamil or cyclosporin A (CsA). Both the CD11bhi CD19+ B cells and the T cells in myeloma PBMCs had active CsA-inhibited dye efflux, but monocytes lacked the ability to efflux dye. Nearly all CD38hi plasma cells from myeloma BM cells retained dye, indicating their lack of a functional transport pump. Thus, PBMC B cells may be the predominant set of drug-resistant tumor cells. Myeloma PBMC B cells were cultured with Adriamycin with or without CsA and drug toxicity evaluated by the induction of apoptosis, using flow cytometry to quantitate DNA disruption. No apoptosis was detectable at 0.01 microgram/mL adriamycin, the in vivo steady-state level, with or without CsA. With 0.1 microgram adriamycin, no apoptosis was detectable in the absence of CsA, but with CsA, 66% of B cells initiated DNA disruption, whereas most T cells were spared. This work suggests that currently used drug dosages are too low to effect P-gp+ B- cell death, even in the presence of CsA. We suggest that blood B cells comprise a highly drug-resistant subset of the myeloma B lineage that escapes conventional chemotherapy and may underlie the almost uniform fatal relapse in myeloma patients.