We report a novel, reproducible methodology which enabled 10 human myeloma cell lines (HMCL) to be obtained from each of 10 tumor samples harvested from 9 patients with extramedullary proliferation. Fresh samples were cultured with interleukin 6 (IL-6) and granulocyte macrophage-colony stimulating factor (GM-CSF) at a high cell density and resulting HMCL growth became progressively dependent on IL-6 alone, no longer requiring GM-CSF. These HMCL, which had the same immunoglobulin gene rearrangements as the patients' original myeloma cells, were designated XG-1 to XG-9. XG HMCL had a plasma cell morphology, expressed plasma cell antigen (Ag), namely cytoplasmic immunoglobulins, CD38, B-B4 Ag, and CD77, and lacked the usual B-cell Ag. They also expressed activation antigens such as CD28 with coexpression of CD28 and its ligand, B7 Ag, in four HMCL. Six HMCL expressed CD40, 4 CD23, and 5 its ligand, CD21. The XG HMCL bore adhesion molecules VLA-4 and CD44 (all 10 HMCL), VLA-5 (7 HMCL), and CD56 (4 HMCL). Finally, cytogenetic study of 8 HMCL indicated a 14q+ chromosome, and t(11,14) translocation was found in 6 of 8 and 5 of 8 HMCL, respectively. The possibility of obtaining malignant plasma cell lines reproducibly from each patient with extramedullary proliferation offers a unique tool for studying the phenotype and abnormalities of the still unidentified tumor stem cell in this disease.