2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an endocrine disrupting environmental pollutant that, among other effects, affects bone tissue. TCDD modulates the transcription of various genes, e.g., CYP1A1, and the present study is a part of a project aiming at developing an in vitro model system for identifying biomarkers specific for dioxin-induced effects in osteoblasts. Osteopontin (OPN) is an adhesion protein, suggested to be important in bone remodeling and our results indicate that TCDD down-regulates the transcription of OPN in the osteoblastic cell line, UMR-106. The present study shows that UMR-106 expresses the AhR and that the expression of CYP1A1 is induced after exposure to TCDD, while down-regulation of OPN is an even more rapid response and a sensitive biomarker to TCDD exposure in this osteoblastic cell line. In conclusion, this osteoblastic cell line may be used as an in vitro model-system for studying dioxin-induced effects on osteoblasts.