The Philadelphia (Ph') chromosome in chronic myelogenous leukemia (CML) results in fusion of the bcr gene and c-abl oncogene, which transcribes into two types of chimeric bcr/abl mRNAs: the L-6 junction and the K-28 junction. By means of a highly sensitive assay, combination of reverse transcription and polymerase chain reaction (RT/PCR), we analyzed 38 blood samples obtained from 31 patients with Ph'-positive CML and two patients with Ph'-negative bcr rearranged CML. Among the 21 samples obtained in chronic phase, eight patients had the L-6 mRNA, 11 had the K-28 mRNA, and two had both the L-6 and K-28 mRNAs. Among the nine samples obtained in blast crisis, four contained the L-6 mRNA, two contained the K-28 mRNA, and three contained both the K-28 and L-6 mRNAs. This finding supports the concept of alternative splicing of bcr/abl mRNAs transcribed in Ph'-positive CML. However, it appears to be a rare event. Of the eight samples obtained from eight patients who had achieved complete cytogenetic remission and negativity for bcr region rearrangement for 6 months to 3 years after recombinant alpha interferon (r alpha-IFN) therapy, all of them showed evidence of minimal residual Ph'-positive clones as detected by the RT/PCR assay. This finding suggests that interferon therapy suppresses the proliferation of the Ph'-positive clones, but it does not completely eradicate the Ph'-positive stem cells.