The recently cloned osteoclastogenesis inhibitory factor, or osteoprotegerin (OPG), has been shown to be a potent inhibitor of osteoclast formation. The inhibition is believed to be mediated through specific binding of OPG to a cell surface ligand on osteoblastic stromal cells. In this report we have studied the effect of the bone resorbing agent prostaglandin E₂(PGE₂) on OPG mRNA levels in primary cultures of human bone marrow stroma cells (hBMSC). PGE₂dose- and time-dependently down-regulated the mRNA levels of OPG, as measured by RNAse protection assay. After 4 hours of stimulation with 1 μM PGE₂, OPG mRNA levels were significantly decreased. The inhibitory effect was seen at and above 1 nM of PGE₂. To elucidate whether the OPG mRNA levels are regulated via the proteinkinase A and/or the proteinkinase C pathways we stimulated cells with either forskolin (FSK) or phorbolic ester (PDbu) respectively. FSK (10 μM) decreased OPG mRNA levels to 50 % of control, whereas PE (10 nM) upregulated the mRNA levels to 250 % of control. These data show that PGE₂down-regulates the expression of OPG mRNA in hBMSC, probably via an increase in cAMP. This mechanism might be involved in PGE₂-induced bone resorption.