A myeloma cell line (KHM-11) was established from the pleural effusion of a patient with IgA-kappa type aggressive myeloma with high serum lactate dehydrogenase who was extremely resistant to vincristin, adriamycin and dexamethasone combination therapy (VAD). The morphology of fresh tumor cells and KHM-11 was plasmablast according to Greipp's criteria. In addition to the expression of regular plasma cell antigens, CD38 and PCA-1, CD45 was found on both fresh cells and KHM-11. Other T- or B-cells antigens, such as CD2, 4, 8, 19, and 20 were negative. Cytoplasmic immunoglobulin kappa light chain in KHM-11 was found by flowcytometry. Southern blot analysis revealed that fresh sample and KHM-11 shared the same immunoglobulin gene rearrangement. IL-6 was found in the culture supernatant of KHM-11, and this supernatant stimulated the growth of this cell line, indicating an IL-6 autocrine mechanism. These findings indicate that KHM-11 is a CD45-positive immature plasma cell line. As far as we know, there is no report of CD45-positive myeloma cell line. KHM-11 should be a useful tool for understanding not only the pathogenesis of aggressive multiple myeloma with high LDH but also for understanding the mechanism which underlies the terminal differentiation of B-cells.