It has been shown that aberrant expression of a transcription factor, GATA-1, leads to maturation arrest and transformation of erythroid cells. We previously reported that a multifunctional protein, YB-1, was expressed strongly in the spleen of a GATA-1 mutant mouse, which was filled with transformed erythroblasts. This finding suggested that YB-1 has roles in erythropoiesis. In this study, we examined in vivo expression of YB-1 messenger RNA (mRNA) in bone marrow erythroid cells and erythroid leukemic cell lines. During erythroid differentiation of erythroid leukemic cell lines, the expression level of YB-1 mRNA was highest at the early phase of differentiation and then decreased. In human bone marrow cells, the in vivo expression level of YB-1 mRNA was higher in glycophorin A-positive cells than in glycophorin A-negative cells. An interesting finding was that expression of YB-1 was higher in erythroblasts in myelodysplastic syndrome-refractory anemia (MDS-RA) than in normal cells. The findings suggested that YB-1 functions in the early stage of erythropoiesis and that aberrant expression of this protein may induce hematological diseases such as MDS.