While tissue-nonspecific alkaline phosphatase (TNSALP) is a well-known indicator of bone formation and all-trans-retinoic acid a key regulator of that process, the relationship between TNSALP and retinoic acid has not yet been clearly described. The aim of the present study was therefore to clarify the mechanism by which retinoic acid modulates expression of TNSALP. After culturing SaOS-2 human osteoblastic osteosarcoma cells in the presence or absence of 10⁻⁶ M all-trans-retinoic acid, real-time RT-PCR confirmed that retinoic acid up-regulates expression of TNSALP mRNA. Notably, this time-dependent induction of TNSALP expression was accompanied by a corresponding increase in detected catalytic activity of the enzyme. When we then isolated the 5′-upstream region of the human TNSALP gene and carried out luciferase assays with a set of deletion mutants, we found that the promoter region contains a retinoic acid response element-like motif. Moreover, electrophoretic mobility shift assays showed that the nuclear extract bound to the motif. It thus appears that retinoic acid regulates the expression of human TNSALP via a retinoic acid response element in the genes promoter region.