GATA-2 is considered to be essential for the development, maintenance, and function of hematopoietic stem cells (HSCs). However, it was also reported that GATA-2 inhibits the growth of HSCs. To examine the role of GATA-2 in the growth of hematopoietic cells, we introduced an estradiol-inducible form of GATA-2 (GATA-2/estrogen receptor [ER]) into interleukin 3 (IL-3)-dependent cell lines, Ba/F3, 32D, and FDC-P1. Estradiol-induced GATA-2 suppressed c-myc mRNA expression and inhibited IL-3-dependent growth in these clones. As for this mechanism, GATA-2 was found to inhibit ubiquitin/proteasome-dependent degradation of p21WAF1 and p27KIp1 and to induce their accumulation by repressing the expression of Skp2 and Cul1, both of which are components of the ubiquitin ligase for p21WAF1 and p27Kip1. Overexpression of c-myc restored the expression of Skp2 and Cull mRNA, reduced the amounts of p21WAF1 and p27Kip1 proteins, and canceled GATA-2-induced growth suppression, suggesting that down-regulation of c-myc expression may be primarily responsible for GATA-2-induced growth suppression. Next, we transduced retrovirus containing GATA-2/ER into murine bone marrow mononuclear cells (MNCs) and stem/progenitor (Sca-1+Lin-) cells. GATA-2/ER suppressed cytokine-dependent growth of MNCs and Sca-1+Lin- cells by about 70%, which was also accompanied by the reduced expression of c-myc, Skp2, and Cul1 mRNA and the accumulation of p21WAF1 and p27Kip1 proteins. In addition, the amount of GATA-2 protein was found to decline in hematopoietic stem/progenitor cells that were promoted to enter cell cycle by the stimulation with cytokines. These results suggest that GATA-2 may regulate expression levels of p21WAF1 and p27Kip1, thereby contributing to the quiescence of hematopoietic stem/progenitor cells.