Two long-term cell lines were established in vitro from the peripheral blood of a patient with plasma cell leukemia: one line with plasma cell proliferation, the other with lymphoblastoid cell proliferation (LCL). The 9-month-old plasma cell line showed the typical morphology of plasmoblasts. The cells neither had B- nor T-lymphocyte characterisitics, were EBV negative, and showed aneuploidy with various marker chromosomes, including the 14 q+ marker. The cytogenetic findings indicate a monoclonal proliferation of the plasmacells. No tumor growth in thymusless nude mice could be induced upon intracranial inoculation with these cells. In contrast, the autologous LCL, cultured after addition of exogenous EBV, showed the characteristic markers of lymphoblastoid cells, with the typical morphology of pear- and handmirror-shaped lymphoblasts, growing in clumps. They had C3- and Fc-receptors, surface-Ig, E-rosette-negativity, a diploid karyotype, and EBV dependent macromolecule synthesis. They lymphoblastoid cells produced intracranial tumors in nude mice in 8 out of 8 attempts.