Osteoclastogenesis inhibitory factor (OCIF) was originally identified as a factor inhibiting osteoclast (OC) formation. The number of OC in rats treated with OCIF decreased, suggesting that OCIF inhibits OC formation in vivo; however, it is also possible that OCIF affects the number of OC by promoting apoptosis of OC. To address this issue, the effects of OCIF on the survival of OC were examined using well established mouse culture systems. OCIF dose-dependently inhibited OC formation in mouse marrow cultures. Addition of OCIF during day 0-3 did not alter the peak levels of OC formation on day 7 and 8. However, the addition of OCIF during day 5 and thereafter resulted in the rapid decrease of the number of OC. OCIF inhibited the survival of OC formed in mouse marrow cultures in dose- and time-dependent manners. The involvement of stromal cells in OC survival was examined using crude and stromal cell-depleted OC populations. OCIF dramatically inhibited the survival of crude OC populations rich with stromal cells. However, in stromal cell-depleted OC populations, OC spontaneously decreased in the absence of OCIF and OCIF did not enhance the decrease further at least up to 48 h. Apoptotic OC were detected in detached cell populations treated with OCIF in mouse marrow cultures and a specific inhibitor for caspase-3 rescued the death of OC. OCIF mutant lacking the death domain homologous regions inhibited OC survival, though the potency was much less than native OCIF. Taken together, OCIF inhibited not only OC recruitment but also OC survival. OCIF inhibited OC survival by interfering the interaction of stromal cells with OC.