Agar overlay cytotoxicity tests of 18 commercial light-activated composite resins were carried out, using L929 mouse fibroblasts in culture, and the difference of adverse reaction to L-cell among certain materials was compared. In addition, the unreacted substances in the cured material, such as Bis-GMA and Triethyleneglycol dimethacrylate, were measured using a high performance liquid chromatograph, and the relationship between the amount of unreacted substance and the injured toxic area (cm2) of L-cell was presented and discussed. From the results, with the exception of a few materials, adequate correlation between both factors was estimated. The injured area of L-cell was enlarged as the unreacted substances in the cured material increased.