In the present study, the in vitro osteogenic differentiating potential of rat dental pulp stem cells was examined. The induction was carried out under the same inducing system as used for bone marrow cells. Alkaline phosphatase (ALP) activity and min-eralization were measured at 1, 2, 3, and 6 weeks after the induction. The level of ALP activity was very high at 1 week after induction and the level was maintained for 3 weeks, however the level of ALP activity was dramatically decreased (to 50%) after 6 weeks. Although mineralization was detected at low levels after 3 weeks, it became expanded to up to 29 nodules mineralized at 6 weeks after induction, as determined by von Kossa staining. It was shown that in vitro osteogenic differentiation of rat dental pulp stem cells was induced following the increase of the ALP activity at an early stage of the induction.