The aorta-gonads-mesonephros (AGM) region of the mouse embryo has been assigned as the origin of definitive hematopoiesis. The transcription factor GATA-2 has specific but unclarified roles in early hematopoiesis. To elucidate the expression profile of GATA-2, we prepared transgenic mouse lines containing the green fluorescent protein (GFP) gene driven by GATA-2 gene regulatory elements. We also prepared a mouse line in which GFP reporter sequences were inserted into the endogenous GATA-2 gene. Both mouse mutants expressed GFP in the early hematopoietic tissues. The CD45 antigen, a marker of hematopoietic cells, was expressed in a small fraction of transgene (TG)–derived GFP⁺ cells. The remaining TG-GFP⁺/CD45^- cells were adherent to plastic and produced CD45⁺ hematopoietic cells abundantly when cultured in vitro. Exogenous expression of GATA-2 in TG-GFP⁺/CD45^- cells from the AGM region inhibited their differentiation into CD45⁺ cells. Loss of GATA-2 function through the disruption of the GATA-2 locus enhanced the earlier emergence of CD45⁺ cells in the yolk sac of the 9.5-day conceptus. These results demonstrated that GATA-2 is expressed in the precursor of hematopoietic cells and works as a gatekeeper to preserve their immaturity. A reduction of GATA-2 expression or activity is required for the differentiation of precursors to hematopoietic cells.