Blood cells derive from a common multipotent stem cell that undergoes successive commitment steps with progressive restriction in differentiation ability. A close developmental relationship exists between the erythroid and megakaryocytic (MK) lineages as both lineages derive from a common bipotent progenitor. The nature of the events governing erythroid-specific or MK-specific programming is yet poorly understood. To approach this issue, we used an ectopic expression of the p210 BCR-ABL oncoprotein in adult primary human CD34+ progenitor cells. In culture conditions favoring the development of both erythroid and MK cells, we observed a major expansion of the erythroid lineage occurring at the expense of the MK lineage. The combined use of a p210BCR-ABL tyrosine kinase inhibitor together with single cell cloning experiments on prospectively isolated CD41+CD42− progenitor cells allowed us to identify that a part of MK progenitor cells were reprogrammed by p210BCR-ABL to take an erythroid fate. A similar lineage switching was seen after p210BCR-ABL expression in the human megakaryoblastic MO7e cell line. Microarrays and semi-quantitative RT-PCR studies showed that expression of p210BCR-ABL was associated with a decreased expression of the FLI-1 transcription factor. Moreover, re-expression of FLI-1 in p210BCR-ABL-transduced MO7e cells that had acquired an erythroid phenotype rescued the MK programming. Finally, a partial inhibition of FLI-1 in normal CD41+CD42− progenitor cells with a shRNAi led to an increased erythroid differentiation at the expense of MK differentiation. These results demonstrate that progenitor cells apparently committed to the MK lineage may be re-specified to the erythroid lineage by an ectopic expression of p210BCR-ABL and that the FLI-1 transcription factor is a key factor in the balance between erythroid versus MK commitment.