This study was aimed to compare the relative cytotoxicity of the five common orthodontic bonding materials (Concise. Mon-lok , Ortho-One, Super C, Transbond) using cell culture technique. DNA synthesis of the fibroblasts was assessed by ^³H-thymidine uptake to evaluate the effect of the bonding materials on the growth of the cells. The human gingival fibroblasts were explanted from the buccal gingiva of 10 year-old girl and cultured in a-MEM/10% FBS/1% antibiotics medium, 37°C, 5% CO_² incuvator. The gingival fibroblasts were tested with the medium into which the bonding materials had been soaked for 1 week. Or the bonding materials were placed on the cells immediately or 2 weeks after polymerization. After 22 hours, ^³H-thymidine was added into the microtest wells and after 24 hours, the uptake of ^³H-thymidine was determined by liquid scintilation counter. The results of this study were as follows. 1. DNA synthesis was significantly decreased with Super C and Transbond than Ortho-One, when treated with medium into which the bonding materials had been soaked for 1 week. 2. DNA synthesis was significantly decreased with Concise, Super C and Transbond than control, when treated immediately after polymerization. 3. DNA syntehsis was significantly decreased with Concise, Super C and Transbond than Ortho-?One, when immediately after polymerization. 4. There was no significant difference in DNA synthesis between the bonding materials, when treated 2 weeks after polymerization.