The finding that large occlusal Class I restorations weaken premolars is not confirmed for natural mandibular molars because no statistically significant difference was found in the mean loads needed to fracture mandibular molars that were unprepared, minimally prepared, or prepared with a large occlusal amalgam restorations. Similarly, there was no statistically significant change in the amount of force necessary to fracture natural mandibular molars prepared with sharp vs round internal line angles (a no. 56 vs a no. 330 bur). Duplication of the experiment in acrylic resin mandibular molars confirmed the results of a previous experiment by Re and Norling as well as the results relating to sharp vs round internal line angles found in this experiment on natural mandibular molars. Acrylic resin teeth of homogeneous shape and substance overcome some of the sampling problems inherent in obtaining suitable human teeth for testing purposes; however, further testing of this material is indicated.