We describe a novel continuous B-cell line (PV-90) derived from a patient with myelodysplastic syndrome (MDS) and originating from spontaneous infection with the Epstein-Barr virus (EBV). The patient progressed to acute myeloblastic leukaemia (AML) 5 months after clinical onset of MDS. PV-90 is of clonal origin as indicated by the presence of immunoglobulin (Ig) gene rearrangements, monoclonal surface immunoglobulins, and a single DNA restriction fragment corresponding to the EBV genomic termini. PV-90 cells also express a number of myelomonocytic markers, including alpha-naphthyl acetate esterase (ANAE), coagulation factor XIII, and CD68 antigen. Moreover, PV-90 cells constitutively express the c-fms proto-oncogene mRNA as the patient's blast cells did. Whereas a trisomy 11 (+11) was found in the patient's bone marrow cells, PV-90 cells had a normal karyotype initially, but at 4 months showed two different and independent chromosomal abnormalities: 90, XX, -Y, -Y, t(9;16) (q11;p13), and 90, XX, -Y, -Y, t(17;18) (p13;q21), the latter possibly involving the p53 (17,p13) and bcl-2 (18, q21) proto-oncogenes. The early development of these chromosomal aberrations is consistent with a genetic instability of PV-90 cells. Expression of bi-lineage markers and genetic instability may suggest that PV-90 cells originated from transformation of a myelodysplastic progenitor cell capable of both myeloid and B-cell differentiation. The PV-90 cell line might be useful in a number of studies, including the possible role of c-fms in cell differentiation, pathogenetic mechanisms of human preleukaemia and lineage promiscuity in acute leukaemia.