An in vitro study was undertaken to evaluate the effect of retention grooves on the gingival marginal seal of class 2 posterior composite resin restorations when filled by the bulk and incremental techniques. Class 2 cavities were prepared in 40 extracted molar teeth. Retention grooves were prepared at the axioproximal line angles in 20 teeth. The teeth were then filled with Herculite posterior composite resin using the bulk and incremental techniques. Microleakage was assessed by radioisotope (I125) diffusion at the gingival margin, quantitatively by radiation counting on an I125 Gamma Counter and qualitatively by autoradiographs. Those class 2 cavities that were prepared with retention grooves and filled in layers showed the least radioisotope diffusion (a mean radiation count of 1588.5), while bulk insertion of the composite resin in cavities without retention grooves showed the most microleakage (mean count of 6092.4).