The t(10;11)(p13-14;q14-21) is a rare but recurring translocation associated with acute lymphoblastic leukaemia (ALL) and acute myeloid leukaemia (AML). Recently the CALM gene was cloned from the t(10;11) breakpoint of U937 and fused to AF10, a putative transcription factor, which had been identified as one of the fusion partners of the MLL gene. In order to define the involvement of these genes in primary leukaemias and cell lines with t(10;11), we analysed the expression of fusion transcripts by reverse transcriptase-polymerase chain reaction (RT-PCR) in five patient samples including ALL, AML and lymphoblastic lymphoma, and three monocytic cell lines (P31/Fujioka, KP-Mo-TS and U937). The CALM-AF10 fusion transcript was detected in all samples; however, the AF10-CALM fusion was not detected in two patient samples and one cell line. In RT-PCR analysis there were six isoforms of the CALM-AF10 fusion transcripts and five of AF10-CALM fusion transcripts. We also detected novel transcripts in U937. Sequence analysis revealed that all these isoforms had in-frame junctions and that some of them resulted from alternative splicing at different exons of CALM and others from different breakpoints at CALM and/or AF10. There were at least two different breakpoints of CALM and three of AF10 gene. Our results suggest that the CALM-AF10 fusion gene is a constant feature and is involved in the pathogenesis of haematological malignancies with t(10;11)(p13-14;q14-21), showing various and often multilineage phenotypes. Thus, t(10;11) needs to be investigated by RT-PCR for identification of the genes involved.