Recent studies have shown that anemia is commonly observed after exposure to pathogens or pathogen-derived products, which are recognized via Toll-like receptor 9 (TLR9). In the current study, we demonstrate that CpG oligodeoxynucleotide-2006, a TLR9 ligand with phosphodiester (PO; 2006-PO) but not with the phosphorothioate backbone, selectively inhibits the erythroid growth derived from human CD34(+) cells. The 2006-PO was internalized by the erythroid progenitors within 30 minutes; however, expression of TLR9 mRNA was not detected in these cells. The 2006-PO directly inhibited burst-forming unit-erythroid growth, resulted in the accumulation of cells in S and G(2)/M phases, and increased cell size and frequency of apoptotic cells. These features were similar to those observed in erythroid progenitors infected with human parvovirus B19 that causes pure red cell aplasia. The consensus sequence of 2006-PO was defined as 5'-GTTTTGT-3', which was located in the P6-promoter region of B19 and inhibited erythroid growth in a sequence-specific manner and down-regulated expression of erythropoietin receptor (EPOR) mRNA and EPOR. B19 genome extracted from serum also inhibited erythroid growth and down-regulated expression of EPOR on glycophorin A(+) cells. These results provide a possible insight into our understanding of the mechanisms of human parvovirus B19-mediated inhibition of erythropoiesis.