ProRoot Mineral Trioxide Aggregate (MTA) has been indicated as a pulp capping material. The purpose of this study was to evaluate the effect of tooth-colored (white) MTA on pulp cell apoptosis and cell cycle. Mouse odontoblast-like cells (MDPC-23) and undifferentiated pulp cells (OD-21) were exposed to 0 to 100 mg MTA for 24 h. Propidium iodide staining followed by flow cytometry demonstrated that MTA did not induce apoptosis of MDPC-23 or OD-21 (p > 0.05). Cell cycle analysis showed that MTA induced a modest (but significant) increase in the percentage of MDPC-23 in the S and G2 phases, and OD-21 in the S phase of cell cycle, as compared to untreated controls (p </= 0.05). In conclusion, MTA induced proliferation, and not apoptosis, of pulp cells in vitro. These findings suggest a potential mechanism to explain the regenerative effect observed in the dentin-pulp complex when MTA was used for direct pulp capping.