Commitment of members of the monocyte/macrophage family to the bone resorptive phenotype, in vitro, requires contact, of these osteoclast precursors, with osteoblasts or related stromal cells. The osteoclast-inductive properties of these stromal cells are typically expressed, however, only in the presence of steroid hormones such as 1,25 dihydroxyvitamin D (1,25D3) and dexamethasone (DEX). To gain insight into the means by which steroid treated accessory cells induce osteoclast differentiation we asked, using differential RNA display (DRD), if gene expression by this stromal cell population differs from that of their untreated, non-osteoclastogenic counterpart. We identified four known genes specifically expressed by 1,25D3/DEX-treated ST2 stromal cells: 1) a family of rat organic anion transporters, 2) Na/K ATPase Γ-subunit, 3) tazarotene-induced gene 2 (TIG2), and 4) prostaglandin G/H synthase I, or cyclooxygenase 1 (Cox-1). The regulation of these genes in 1,25D3/DEX-treated ST2 cells was demonstrated by Northern blot analysis of treated (osteoclast-supporting) and untreated (non-osteoclast-supporting) ST2 cells; the genes have a limited and specific tissue mRNA expression pattern. Northern blot analysis of treated and untreated ST2 cell total RNA using either a DRD-derived Cox-1 cDNA or a Cox-1 specific oligonucleotide confirmed the steroid regulation of Cox-1 mRNA. Surprisingly, there is no detectable expression by untreated or steroid exposed ST2 cells, of Cox-2, the classical regulated cyclooxygenase isoform. In contrast to 1,25D3/DEX, serum treatment rapidly induces Cox-2 mRNA, substantiating the capacity of ST2 cells to express the gene. These data establish that steroid induction of the osteoclastogenic properties of stromal cells is attended by Cox gene expression, a phenomenon consistent with the capacity of eicosinoids to impact the resorptive process. The response of osteoclast-supporting ST2 cells to 1,25D3/DEX treatment may be one prostaglandin-mediated event which specifically involves Cox-1 regulation.