Prostaglandin plays a significant role in the local control of bone metabolism associated with periodontal disease. ${Delta}^{12}-PGJ_2$ is a natural $PGD_2$ metabolite that is formed in vivo in the presence of plasma. It is known for ${Delta}^{12}-PGJ_2$ to stimulate calcification in osteoblastic cells. Bone morphogenetic protein(BMP) stimulated osteoblastic differentiation in various types of cells and greatly enhanced healing of bony defects. The purpose of this study was to evaluate the effect of rhEMP-2 on ${Delta}^{12}-PGJ_2$ induced osteoblastic differentiation and mineralization in vitro. A human osteosarcoma cells line Saos-2 were cultured. In the test groups, 10-7M of ${Delta}^{12}-PGJ_2$ or mixture of 10-8M of ${Delta}^{12}-PGJ_2$ and 100ng/ml of rhBMP-2 or 100ng/ml of rhEMP-2 were added to culture media. After 1 day, 2 days and 4 days of culture period, the cell number was measured. Alkaline phosphatase activity was measure at 3 days. Reverse transcription polymerase chain reaction(RT-PCR) was performed to determine the expression of mRNA of bone matrix protein at 8 hours, 1 day and 7 days. The ability to produce mineralized nodules in rat osteoblasts(MC3T3-E1) was evaluated at 21 days. The results were as follows : 1. rhEMP-2 or mixture of rhBMP-2 and ${Delta}^{12}-PGJ_2$ inhibited cell proliferation of human osteosarcoma cells. 2. rhEMP-2 or mixture of rhBMP-2 and ${Delta}^{12}-PGJ_2$ stimulated alkaline phosphatase activity significantly higher than ${Delta}^{12}-PGJ_2$ alone. 3. rhBMP-2 or mixture of rhEMP-2 and ${Delta}^{12}-PGJ_2$ stimulated mineralization compared to ${Delta}^{12}-PGJ_2$ alone. 4. mRNA of alkaline phosphatase, BMP-2, cbfa 1, Type I collagen were detected in the group treated with ${Delta}^{12}-PGJ_2$/rhBMP-2, rhBMP-2 alone, ${Delta}^{12}-PGJ_2$ alone. These results show that mixture of ${Delta}^{12}-PGJ_2$ and rhBMP-2 causes more bone formation than ${Delta}^{12}-PGJ_2$ alone while the bone formation effects of mixture of ${Delta}^{12}-PGJ_2$ and rhBMP-2 are less than those of rhBMP-2 alone. Further researches would be necessary to clarify the interactions of these agents.