microRNAs (miRNAs) are non-coding RNAs, and are negatively regulating the gene functions through translational repression or digesting mRNAs by targeting 3'-UTR in messenger RNAs (mRNAs). To clarify roles of miRNAs in normal and pathological erythropoiesis, we analyzed the expression of miR-451, miR-155, miR-221 in normal human erythroid colony forming cells (ECFCs) and circulating red blood cells using quantitative real-time polymerase chain reaction. Remarkable down-regulation of miR-155 (about 200-fold) and up-regulation of miR-451 (270-fold) were observed during 12 days of ECFC cultures. miR-221 was down-regulated moderately (20-fold). Since mature erythroid cells expressed miR-451 at high levels, we analyzed the content of miR-451 in fractionated peripheral blood cells obtained from normal subjects (n= 3), and found that the expression level of miR-451 in red blood cells was about 10(4)-fold more than in granulocytes and mononuclear cells. Analysis of packed red cells from normal subjects (n=3) and patients with hematological disorders (n=16), showed that there was no significant difference of the expression levels of miR-451 between normal and patients. In conclusion, expression profiles of miR-155 and miR-451 are stage-specific during normal erythropoiesis in which miR-155 is down-regulated and miR-451 is up-regulated. A high expression of miR-451 is specific to red blood cells among circulating blood cells. These observations suggest that an analysis of microRNAs is a new relevant approach to understand underlying mechanisms of hematological disorders.