We have studied the capacity of peripheral blood cells from 26 chronic B cell leukaemias to proliferate continuously in culture; 72 attempts to establish cell lines were made. The cells were treated in vitro with or without stimulating agents: Epstein-Barr virus (EBV) and/or phorbol-ester (TPA) were the most frequently used. Fourteen cell lines of continuous growth were established from cells from 11 patients, but only four of these were proven to be derived from the original leukaemic cells. Only in the latter four lines were the karyotypic abnormalities and the patterns of immunoglobulin (Ig)-gene rearrangements identical to those found in the patients' leukaemic cells. On the other 10 lines, five had both kappa- and lambda-producing cells, and the remaining five, despite showing light-chain restriction, were proved to be non-leukaemic clones by comparing the Ig-gene rearrangement patterns before and after culture. Three of the four leukaemic cell lines (JVM-2, JVM-3 and JVM-13) were induced by EBV + TPA and derived from prolymphocytic leukaemia (PLL) cases; the fourth (JVM-14) originated from a case of chronic lymphocytic leukaemia (CLL) with increased percentage of prolymphocytes whose cells were stimulated in vitro with EBV. The immunophenotype of the three PLL lines is more mature than that of the original prolymphocytes, as shown by a reduction in surface-Ig and FMC7 expression, enhancement of cytoplasmic-Ig and increase in CD38- and transferrin receptor-positive cells. The cells from line JVM-14 retained the CD5-antigen, a marker of CLL. This study suggests that PLL and some CLL clones are arrested at a stage of maturation ideally suited to be triggered to continuous proliferation in culture. The presence of consistent chromosomal abnormalities in PLL may offer an alternative explanation for the greater proliferative potential of these cells in vitro.