Estrogen deficiency causes bone loss as a result of accelerated osteoclastic bone resorption. It also has been reported that estrogen deficiency is associated with an increase in the number of pre‐B cells in mouse bone marrow. The present study was undertaken to clarify the role of altered B lymphopoiesis and of the receptor activator of nuclear factor‐κB ligand (RANKL), a key molecule in osteoclastogenesis, in the bone loss associated with estrogen deficiency. In the presence of prostaglandin E₂ (PGE₂), the activity to form tartrate‐resistant acid phosphatase (TRAP)‐positive osteoclast‐like cells was significantly greater in bone marrow cells derived from ovariectomized (OVX) mice than in those from sham‐operated mice. Northern blot analysis revealed that PGE₂ increased the amount of RANKL messenger RNA (mRNA) in bone marrow cells, not only adherent stromal cells but nonadherent hematopoietic cells; among the latter, RANKL mRNA was more abundant in OVX mice than in sham‐operated mice and was localized predominantly in B220⁺ cells. Flow cytometry revealed that most B220⁺ cells in bone marrow were RANKL positive and that the percentage of RANKL‐positive, B220^low cells was higher in bone marrow from OVX mice than in that from sham‐operated mice. The increase in the expression of RANKL and the percentage of these cells in OVX mice was abolished by the administration of indomethacin in vivo. PGE₂ also markedly increased both the level of RANKL mRNA and cell surface expression of RANKL protein in the mouse pre‐B cell line 70Z/3. Finally, osteoclastogenic response to PGE₂ was reduced markedly by prior depletion of B220⁺ cells, and it was restored by adding back B220⁺ cells. Taken together with stimulated cyclo‐oxygenase (COX)‐2 activity by tumor necrosis factor α (TNF‐α) and interleukin‐1 (IL‐1) in estrogen deficiency, these results suggest that an increase in the number of B220⁺ cells in bone marrow may play an important role in accelerated bone resorption in estrogen deficiency because B220⁺ cells exhibit RANKL on the cell surface in the presence of PGE₂, thereby leading to accelerated osteoclastogenesis.