We developed an efficient differentiation induction system from mouse embryonic stem (ES) cells into blood cells by coculture on a novel stromal cell line named OP9, in order to analyze molecular mechanisms of hematopoietic cell development and differentiation. ES cells could give rise to adult type definitive erythrocytes, myeloid and B lineage cells via multipotential hematopoietic precursor cells, when the cells were simply cocultured with the OP9 stromal cells. The temporal pattern of the appearance of erythroid lineage cells during the differentiation induction was very similar to that detected in mouse ontogeny. This differentiation induction method should facilitate to dissect the function of erythroid transcription factors during erythroid lineage cell development.