Lateral mesoderm-derived hemogenic endothelial cells are known to originate the definitive hematopoietic lineage in mouse embryogenesis. The developmental process of the definitive hematopoietic lineage can be recapitulated by inducing differentiation of mouse embryonic stem (ES) cells in a co-culture system with OP9 stromal cells. However, the signaling molecules that can modulate the development of the definitive hematopoietic lineage in the OP9 co-culture system have yet to be identified. Here we report that activin A enhanced the hematopoietic potential of endothelial cells derived from ES cells in the OP9 co-culture system. Activin A in combination with OP9 cells augmented development of Flk-1(+) PDGFR alpha(+) early mesodermal cells and Flk-1(+) PDGFR alpha(-) lateral mesodermal cells from ES cells. These Flk-1(+) mesodermal cells further differentiated into CD41(+) endothelial cells, which preferentially possessed high hematopoietic potential. Furthermore, Flk-1(+) PDGFR alpha(+) cells but not Flk-1(+) PDGFR alpha(-) cells produced hematopoietic progenitors with a bimodal pattern when cultured as an aggregate with OP9 cells. Our results suggest that activin A in combination with OP9 cells facilitates differentiation of ES cells to Flk-1(+) mesodermal cells, which encompass various precursors that separately contribute to the development of hematopoietic lineages. (C) 2015 Elsevier Inc. All rights reserved.