The purpose of the present study was to evaluate the physicochemical and cytologic properties of mineral trioxide aggregate (MTA) mixed with distilled water and Na(2)HPO(4) buffer solution. The MTA setting time and pH value were evaluated. An MTA micrograph on scanning electron microscopy (SEM) was observed. Mouse fibroblasts (an L929 cell line) were used to test the toxicity of MTA after the first and seventh day of treatment by mitochondrial colorimetric assay. The results show the Na(2)HPO(4) buffer group reduced the MTA setting time, and the pH value as in the distilled water group is similar with the Na(2)HPO(4) buffer group. The mixed MTA's XRD produced similar peaks of the distilled water and Na(2)HPO(4) buffer solution groups. The L929 cell survival rate of distilled water and H(2)PO(4) buffer solution groups did not exhibit any significant difference (p < 0.05). There are differences in SEM observations both of the MTA surface and of the cells in culture on the surface of the MTA with Na(2)HPO(4) versus distilled water. The results suggest that 15% Na(2)HPO(4) buffer can be successfully used as an accelerator of MTA.