In previous studies, we demonstrated an over-expression of the dominant-negative isoform of the transcription factor Ikaros, Ik-6, in patients with B-cell malignancies, including blast crisis of chronic myelogenous leukaemia and acute lymphoblastic leukaemia. To investigate the consequence of over-expression of Ik-6 in B cells, we constructed Ik-6 transfectants of the FDH-1 and Ramos cell lines. FDH-1, which was established from a patient with early pre-B acute lymphoblastic leukaemia, undergoes apoptosis with dexamethasone treatment, whereas Ramos undergoes apoptosis following anti-IgM antibody treatment. Compared with the wild type, the over-expression of Ik-6 rendered the FDH-1 and Ramos transfectants resistant to dexamethasone-induced and anti-IgM-induced apoptosis respectively. An immunoblotting study demonstrated bcl-2 upregulation in anti-IgM-induced Ramos Ik-6 transfectants, but not in FDH-1 Ik-6 transfectants. Further investigations of the mechanism of leukaemogenesis associated with the over-expression of Ik-6 are warranted.