Osteoclastic bone resorption increases at the site of bone metastasis, but little is known about how tumor cells induce osteoclast (OC) recruitment in the bone marrow microenvironment. To clarify this point, we examined the effects of various mouse tumor cells on OC recruitment using cocultures of tumor cells and mouse marrow cells. The mouse mammary tumor cell lines, MMT060562 (MMT), BALB/c-MC, Jyg-MC(A), or other nonmammary tumor cell lines, LLC and B16, were cocultured with mouse marrow cells, and OC recruitment from marrow cells was determined by counting the number of tartrate-resistant acid phosphatase-positive multinucleated cells (TRAP(+) MNCs) formed. Of the tumor cells examined, MMT and BALB/c-MC stimulated OC formation, but other tumor cells did not. OC formation with MMT was dependent on the number of MMTs inoculated, and only ten cells per well were sufficient to induce OC development. OCs appeared on day 4, and the number reached a maximum on days 5-8 and decreased thereafter. TRAP(+) MNCs induced by MMT satisfied the major criteria of OCs, such as the presence of calcitonin receptors and the ability to resorb calcified tissues. The majority of OCs were formed adjacent to the stromal cells, which were positive for alkaline phosphatase. When spleen cells were cocultured with MMT, no OCs were formed. In contrast, when osteoblastic cells were added to cocultures of spleen cells and MMT, many OCs were formed. The cultured media (CM) of MMT induced OC formation in mouse marrow cultures. Neither parathyroid hormone-like nor interleukin 1-like activity was present in the CM. MMT constitutively produced prostaglandin E2 (PGE2) and OC formation in cocultures was completely inhibited by indomethacin. Fractionation of the CM of MMT by ultrafiltration indicated that the OC-inducing activities were present not only in the fraction with molecular weight below 3 kDa but also in the fraction with molecular weight above 3 kDa. OC-inducing activity with high molecular weight was eluted around 50 kDa by Bio-Gel P-60 column chromatography. The active fractions also possessed leukemia inhibitory factor (LIF) activity, and OC-inducing activity of the peak fraction was inhibited in the presence of anti-LIF neutralizing antibody. The results of this study indicated that MMTs release PGE2 and LIF, which in turn stimulate OC formation via a stromal cell-dependent pathway. These culture systems will help to clarify the mechanisms by which tumor cells induce OC formation in a bone marrow microenvironment.