The microgravity environment of spaceflight leads to a series of changes in the human blood system. The aim of the present study was to examine the influence of simulated microgravity on the differentiation of CD34+ cells and to explore whether transcription factor GATA-1, required for the terminal differentiation of committed erythroid progenitor cells, is involved in this process. CD34+ cells were cultured in the simulated microgravity conditions created by a rotary cell-culture system (RCCS). The effects of simulated microgravity on the differentiation and apoptosis of CD34+ cells were analyzed using flow cytometry and propidium iodide (PI) staining, respectively. Expression of GATA-1 mRNA in CD34+ cells was determined by real-time quantitative PCR. In the RCCS group, GlyA+ (glycophorin A) expression was lower and CD33+ expression higher than in the 1-g liquid control group (22.21% +/- 3.02% and 60.05% +/- 3.08%, vs. 52.12% +/- 1.92% and 18.87% +/- 1.41%, respectively). The proportion of differentiated cells in the 1-g methylcellulos e group (Gly+% = 54.39% +/- 2.86%, CD33+% = 21.09% +/- 3.19%) was similar to that in the 1-g liquid control group. As shown by real-time quantitative PCR, the relative expression of GATA-1 mRNA in the RCCS group was only 20% of that in the -g control group. CONCLUSIONSs: The differentiation of CD3+ cells, and especially erythroid differentiation, was inhibited by simulated microgravity by a mechanism that appears to involve the suppression of GATA-1 mRNA expression. The results of this study may be useful in understanding the critical effect of simulated microgravity on the pathogenesis of space anemia.