Leukaemic cells from 2 patients with B-prolymphocytic leukaemia were immortalized in vitro by means of Epstein-Barr virus and phorbol-ester TPA. The resulting cell lines, named JVM-2 and JVM-3, have been growing continuously in liquid culture for more than one year. JVM-2 is characterized cytogenetically by t(11;14)(q13; q32), and JVM-3 by trisomy 12. The immunoglobulin (Ig) heavy- and light-chain genes showed the same pattern of rearrangement in both lines as in the original prolymphocytes from each case. The cells from these lines showed a spectrum of morphological and immunological features corresponding to different stages of B-cell maturation. The expression of Ig, IgM-lambda in JVM-2 and IgMD-K in JVM-3, changed from a predominantly membrane pattern in the original cells to a cytoplasmic one in the cell lines. By comparison with their original progenitors, the cells from both lines showed reduced reactivity with the monoclonal antibody (MAb) FMC7, and increased expression of the antigens recognized by the MAbs OKT10, alpha-Tac, FMC53 and Ki-I. The availability of cell lines from this rare type of lymphoid leukaemia offers a potential tool for the study of molecular events associated with the expression of Ig and other antigens by neoplastic cells.