Philadelphia chromosome positive acute lymphocytic leukemia and chronic myelogenous leukemia are strongly associated with two distinct forms of bcr-abl chimeric protein, known as P190 and P210, respectively. By studying cDNA clones obtained from the cell line KBM-5, we identified two new bcr-abl transcripts. These are formed by alternative splicing of at least two exons to the known bcr exon 2. One novel transcript can encode a protein kinase of approximately 190 kd, while the other can direct the synthesis of a larger protein whose amino terminus remains to be defined. The alternative exons can be spliced also to the two normal bcr transcripts, reflecting the activation of a cryptic promoter. These messages were present at low abundance in two cases of blastic crisis but were not detected in the chronic phase. It is conceivable that the proteins encoded by the new bcr-abl mRNAs are involved in the transformation to the acute phase in some cases of chronic myelogenous leukemia.