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Beta tricalcium phosphate (β-TCP) has been developed as one of the carriers of recombinant human bone morphogenetic protein (rhBMP). However, it is not known whether the particle size of β-TCP is related to its resorption rate and the degree of bone formation. The purpose of this study was to evaluate the effect of using β-TCP with different particle sizes on the ability of rhBMP-4 to enhance bone formation in the rat calvarial defect model.Calvarial, 8-mm-diameter, critical-size defects were created in 100 male Sprague-Dawley rats. Five groups of 20 animals each received either rhBMP-4 (2.5 μg) using β-TCP with a particle size of 50 to 150 mm, rhBMP-4 (2.5 μg) using β-TCP with a particle size of 150 to 500 mm, a β-TCP control with a particle size of 50 to 150 mm, a β-TCP control with a particle size of 150 to 500 mm, or a sham-surgery control, respectively, and were evaluated by measuring their histologic and histometric parameters following a 2- and 8-week healing interval.There were no significant differences in the defect closure, new bone area, or augmented area between either the two rhBMP-4/β-TCP groups or between the two β-TCP control groups at 2 and 8 weeks.rhBMP-4 combined with either small- or large-particle β-TCP had a significant effect on the induction of bone formation compared to either a small- or large-particle β-TCP control or a sham-surgery control. Within the parameters of this study, varying the particle size of β-TCP did not seem to have a significant effect on bone formation. J Periodontol 2006;77:765-772.