Radiopacity is needed in order to facilitate diagnosis of polymeric appliances, which may be dislodged and become impacted in the upper respiratory or digestive tracts. In order for a stable, optically transparent, radiopaque material to be provided, heavy-metal compounds were investigated which we had previously shown to form homogeneous structures with methyl methacrylate-based systems. It was found that, when present in PMMA at 11 to 14%, several compounds of either bismuth or uranium or 35% of an organo-zirconium compound impart radiopacity equivalent to that of aluminum. A low level of cytotoxicity and lack of mutagenicity indicated that a high level of biocompatibility can be expected. Processing characteristics are somewhat altered, but formulations satisfactory for use in various dental devices were found.