T(3;14)(q27;q32) is frequently detected in B-cell non-Hodgkin's lymphomas, especially the diffuse large cell type and the follicular type. The BCL6 gene encoding a putative transcriptional factor which resides on 3q27 rearranges to the immunoglobulin heavy chain (IgH) gene on 14q32 in this chromosomal translocation. The upstream regulatory region of the BCL6 gene is replaced by the IgH gene. Deregulation of the BCL6 gene may contribute to tumourigenesis of these diseases. The rearrangement between the IgH and BCL6 genes generates chimaeric transcripts in which the joining (J) region of the IgH gene fuses to exon 3 of the BCL6 gene. We established a method to detect these chimaeric transcripts by reverse transcriptase polymerase chain reaction (RT-PCR) using the consensus sequence of the J region and the sequence of exon 3 of the BCL6 gene as primers. Using the semi-nested RT-PCR method and a cell line carrying t(3;14)(q27;q32), we detected one lymphoma cell among 10 000 background cells. We detected these chimaeric transcripts in two out of 13 clinical samples by this method. This method can detect t(3;14)(q27;q32) easily, whereas this alteration is frequently overlooked by routine karyotype analysis. Since this technique is sensitive enough to detect a small number of lymphoma cells with this genetic abnormality, it could be employed to detect contaminating lymphoma cells in bone marrow and peripheral blood and minimal residual diseases.