Calcium hydroxide has been the material of choice for apexification. The purpose of this study was to compare the efficacy of osteogenic protein-1 and mineral trioxide aggregate with that of calcium hydroxide in the formation of hard tissue in immature roots of dogs. Sixty-four roots of premolars were used. After induction of periradicular lesions, the canals were debrided and filled with calcium hydroxide for 1 wk. After the removal of calcium hydroxide, the root canals received one of the treatment materials in a balanced design. The animals were euthanized 12 wk later. The degree of hard tissue formation and amount of inflammation were evaluated histomorphically. Data were statistically evaluated using ANOVA, chi 2, and Kruskal-Wallis. Mineral trioxide aggregate produced apical hard tissue formation with significantly greater consistency. The difference in the amount of hard tissue produced among the three test materials was not statistically significant. Furthermore, the degree of inflammation was not significantly different between the various test groups.