Methods of detecting minimal residual disease (MRD) in chronic myeloid leukemia (CML) include chromosome analysis, Southern blotting, polymerase chain reaction (PCR) and fluorescent in situ hybridization (FISH) techniques. We report a novel method to detect intracellular messenger RNA (mRNA) by combining the techniques of reverse transcription (RT) and PCR performed directly inside the cells, without extraction of the nucleic acid. We applied this method, which we call "in-cell RT-PCR", to detect hybrid BCR/ABL transcript within single cells. After cellular permeabilization and fixation of single cells in suspension, the neoplastic mRNA was reverse transcribed into cDNA, and the cDNA was amplified by PCR with fluorescent primers, specific for bcr/abl. Flow cytometry was used to detect cells positive for the amplified DNA within the cell cytoplasm. After transferring the amplified cells onto slides by cytospin, the positive cells for BCR/ABL cDNA were observed by fluorescent microscopy. The technique was capable of detecting low abundancy signals and distinguishing different levels of gene expression. The amplification products were found in the cells and supernatants. The distribution was critically affected by the protease digestion condition. The specificity of amplification was confirmed by a nested RT-PCR of BCR/ABL performed on extracted mRNA from the same sample, and by reamplification of supernatants. We have used the technique to study 10 Ph+ CML patients and three normal subjects as controls. Four patients were 100% Ph+ at diagnosis time and RT-PCR+ at cytogenetic and molecular analysis, respectively. In-cell RT-PCR showed that the residual non-neoplastic cells could be observed in all cases. In two patients undergoing interferon-alpha (IFN-alpha) therapy and in four bone-marrow transplanted patients, the in-cell RT-PCR was used to compare the level of Ph+ positivity detected by cytogenetic analysis with the number of cells expressing BCR/ABL transcript. In this manner, we could estimate the MRD. Our preliminary application of the technique suggests that it is capable of accurately identifying cells transcribing bcr/abl, and that it may have significant clinical applications in the detection of MRD.