Class V abrasion and erosion lesions restored with composite resin seem to be more susceptible to microleakage, because the gingival margin is usually placed in cementum and / or dentin. The purpose of this study was to evaluate the microleakage, in vitro, of two new dentinal bonding systems along with a positive and negative control. This study was accomplished using extracted human teeth and a calcium-45 radioisotope technique. The teeth were examined for microleakage at periods of 1 week, 6 months, and 1 year. The results at all time intervals indicated that the cavities restored with the new bonding agents and their respective composite resins leaked more than did the negative control, glass-ionomer cement.