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The characteristics of a human cell line (LP-1) derived from the peripheral blood of a patient with IgG-lambda myeloma in leukemic transformation are described. The cells resemble immature plasma cells in that they exhibit a membrane phenotype that is intermediate between late B lymphocytes and plasma cells, even though they secrete IgG- lambda chains. Treatment of LP-1 cells with 12–0 tetradecanoylphorbol- 13-acetate (TPA) or pokeweek mitogen (PWM) induces the appearance of surface markers and ultrastructural features typical of mature plasma cells but does not affect their proliferative activity. Molecular analysis of the cell line showed an increased expression of the c-myc protooncogene and the presence of abnormally sized transcripts. Conventional cytogenetics and pulsed-field gel electrophoresis showed no structural rearrangements of the c-myc gene, suggesting that the abnormal c-myc expression may be due to point mutations or small deletions within the gene. The LP-1 cell line is a useful model in which to study the process of B-cell maturation; such study may lead to the uncovering of unusual mechanisms of c-myc activation. Furthermore, the LP-1 cell is a potential partner in the generation of human hybridomas.