In studying a series of synovial biopsy specimens from patients with loose joint replacements, we were able to differentiate reliably between polymethylmethacrylate and ultra high-molecular-weight polyethylene by observing alterations in their birefringence with changes in temperature. The glass transition temperature of polymethylmethacrylate (when it softens from a glass-like to a rubber-like state) is about 105 degrees centigrade. Polyethylene melts at 135 degrees centigrade. The birefringence in polyethylene returns after it has been melted and then cooled; this is not true of methylmethacrylate fragments heated above their glass transition temperature. When cooled, methacrylate fails to regain its birefringence. In addition, free particles of the two plastics were studied in vitro to validate this method of differentiation. We recommend its use routinely in histological studies of patients with loosening.