To relate the characteristics of rotary instruments to the sur face they produce, crylic resin, Molloplast B, and Novus were investigated with energy dispersive analysis and scanning electron micjroscopy (secondary and backscatter images) before and after machining. The chemical composition of cutting instruments, material surfaces, and residues was identified. Machined debris embedded in Molloplast B after machining with the Molloplast stone was found to contain a mean lead content of 45%. High concentrations of barium sulphate were discovered on the arbor band-machined surface of Novus. These differences were related to clinically appropriate instrumentation, and, therefore biocompatability studies that intimately relate to the in vivo situation should be considered for new materials.