This study evaluated the effects of wear of human tooth enamel on the metal-porcelain interface. Failure in the porcelain half of the specimens occurred only in specimens where the metal angle was 60 degrees. There was failure in one of four of the noble metal alloy specimens. With the base metal alloy specimens, in the first group two specimens showed cracks in the preparation stage while the other two showed no sign of cracks either before or after testing. In a second group of four base metal alloy specimens with a 60-degree angle, all four specimens showed cracks in the porcelain after glazing. These specimens were repaired by grinding out the cracks and filling them with porcelain. After wear-testing, all four specimens developed cracks. The data indicate that the acute angle of metal (60 degrees) at the metal-porcelain interface was more conducive to crack formation than were the other two angles tested. Therefore, in the case of a metal-ceramic restoration, with the occlusion on the metal, an acute angle of metal would not be appropriate. Furthermore, under clinical circumstances cracks that occur after glazing, as arose with the last four base metal alloy specimens (60 degrees), probably should not be ground out and refilled with porcelain. The results of this study indicate that such a practice in a clinical situation could lead to eventual fracture of the porcelain. The porcelain half of the specimens had a deeper and wider wear track than its metal counterpart, which indicated that the porcelain is less resistant than the metal to wear by enamel.(ABSTRACT TRUNCATED AT 250 WORDS)