Core binding factor alpha 1 (Cbfa1) is an osteoblast-specific transcription factor essential to develop a mature osteoblast phenotype. However, its exact role in the signaling of various osteotropic-differentiating agents is still unclear. In this study, we assessed the effects of 1,25-(OH)(2)-D3 (D3), ascorbic acid, bone morphogenetic protein-2 (BMP-2), dexamethasone (Dex), and transforming growth factor-beta (TGF-beta) on Cbfa1 and osteocalcin (OCN) mRNA steady state levels (by semiquantitative RT-PCR) in an in vitro model of osteoblast differentiation. TGF-beta increased Cbfa1 mRNA levels in normal primary human osteoblasts (pHOB) by 2.6-fold in a time-dependent fashion with maximum effect on day 28 (P < 0.001). Similarly, the glucocorticoid Dex enhanced Cbfa1 gene expression by pHOB in a time-dependent fashion by up to 4.6-fold (P < 0.001). In contrast, Dex inhibited OCN gene expression levels by 68% (P < 0.01). Treatment with BMP-2 resulted in an earlier enhancement of Cbfa1 and led to a 4.2-fold increase with a maximum on day 21 (P < 0.001). Ascorbic acid did not modulate Cbfa1 and OCN gene expression. The effect of vitamin D (D3) on Cbfa1 mRNA expression was influenced by the duration of treatment, being inhibitory after 1 h and having a stimulatory effect after 48 h. Time course experiments indicated a stimulatory effect of D3 on Cbfa1 mRNA levels (by 2.5-fold after 48 h; P < 0.01). Analysis of the late cellular differentiation marker osteocalcin revealed that D3 increased OCN gene expression by 14-fold (P < 0.001). In conclusion, in normal primary human osteoblasts, the rapid and pronounced increase of OCN after treatment with D3 seems not to be mediated by Cbfa1. These data imply that Cbfa1 gene expression is differentially regulated by various osteoblastic differentiating agents and is dependent on the stage of maturation.