Aryl hydrocarbon receptor (AhR) ligands are environmental contaminants found in cigarette smoke and other sources of air pollution. The prototypical compound is TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), also known as dioxin. There is an increasing body of knowledge linking cigarette smoking to osteoporosis and periodontal disease, but the direct effects of smokeassociated aryl hydrocarbons on bone are not well understood. Through the use of resveratrol (3,5,4 -trihydroxystilbene), a plant antifungal compound that we have recently demonstrated to be a pure AhR antagonist, we have investigated the effects of TCDD on osteogenesis. It was postulated that TCDD would inhibit osteogenesis in bone-forming cultures and that this inhibition would be antagonized by resveratrol. We employed the chicken periosteal osteogenesis (CPO) model, which has been shown to form bone in vitro in a pattern morphologically and biochemically similar to that seen in vivo, as well as a rat stromal cell bone nodule formation model. In the CPO model, alkaline phosphatase (AP) activity was reduced by up to 50% (P<0·01 vs control) in the presence of 10 9 M TCDD and these effects were reversed by 10 6 M resveratrol (P<0·05 vs TCDD alone). TCDD-mediated inhibition of osteogenesis was restricted primarily to the osteoblastic differentiation phase (days 0-2) as later addition did not appear to have any effects. Message levels for important bone-associated proteins (in the CPO model) such as collagen type I, osteopontin, bone sialoprotein and AP were inhibited by TCDD, an effect that was antagonized by resveratrol. Similar findings were obtained using the rat stromal bone cell line. TCDD (at concentrations as low as 10 10 M) caused an approximately 33% reduction in AP activity, which was abrogated by 3·5 10 7 M resveratrol. TCDD also induced a marked reduction in mineralization (75%) which was completely antagonized by resveratrol. These data suggest that AhR ligands inhibit osteogenesis probably through inhibition of osteodifferentiation and that this effect can be antagonized by resveratrol. Since high levels of AhR ligands are found in cigarette smoke, and further since smoking is an important risk factor in both osteoporosis and periodontal disease, it may be postulated that AhR ligands are the component of cigarette smoke linking smoking to osteoporosis and periodontal disease. If so, resveratrol could prove to be a promising preventive or therapeutic agent for smoking-related bone loss.