We established three sister cell lines, NALM-30, NALM-31 and NALM-32, with biphenotypic features carrying myeloperoxidase mRNA and protein with complex Philadelphia (Ph) chromosome, t(9;22;10)(q34;q11;q22), from a patient with Ph-positive acute leukemia in relapse. Epstein-Barr virus nuclear antigen was negative. The morphological appearance of the cell lines is that of immature lymphoid cells. Expression of myeloid- and lymphoid-associated surface membrane antigens on these cells was detected allowing for the classification of "biphenotypic" leukemia. Immunophenotypically, the established cell lines reported here fulfill the European Group for the Immunological Characterization of Leukemias (EGIL) criteria for B-lineage derivation, however, surface and cytoplasmic immunoglobulin chains were negative. Whereas TGF-beta R (CD105), MCSFR (CD115), SCFR (CD117), IL-4R/IL-13R (CD124) and IL-6R (CD126) were not expressed, the cell lines were mostly positive for IFN-gamma R (CD119), IL-7R (CD127) and FLT-3R (CD135). The NALM-30, NALM-31 and NALM-32 cell lines together with their serial sister cell lines NALM-27 and NALM-28 which were established from the same patient at diagnosis provide unprecedented opportunities for studying a multitude of biological aspects related to normal and neoplastic immature B-lymphocytes.