Dendritic cell (DC) development begins in the bone marrow and immature progenitors reach their sites of residence in lymphoid organs. The mechanism of DC development in the bone marrow and in peripheral lymphoid organs is poorly understood. Here, we examined the effects of synthetic oligodeoxynucleotides containing a CpG motif (CpG-ODNs) on the development of DC from the bone marrow cells. Approximately 15% of bone marrow cells expressed CD11c surface antigen in the in vitro culture for 8 days in the presence of IL-3. However, the addition of a phosphorothioate-modified CpG-ODN (PTO-CpG-ODN), but not a phosphodiester-modified ODN (PO-CpG-ODN), suppressed the expression of CD11c surface antigen. Also, we examined the effects of CpG-ODNs on the maintenance of DCs resident in the gastrointestinal lymphoid tissues, where immature progenitors may be challenged by a variety of ODN derived from microflora. The population of CD11c(+)B220(int)Gr-1(low) cells decreased by the addition of PTO-CpG-ODN when the lymphocytes from mesenteric lymph nodes as well as spleen are cultured for 2 days in the presence of GM-CSF. In contrast, this population increased in the case of the lymphocytes from Peyer's patches. It thus follows that PTO-CpG-ODN plays a regulatory role in the differentiation of bone marrow cells into DCs and in the functional maintenance of DCs at peripheral lymphoid organs.