T he increase in the number of carriers of hepatitis and acquired immunodeficiency syndrome (AIDS) viruses has caused alarm for many dentists and dental technicians. According to a recent estimate, there are 200,000 new carriers of hepatitis B in the United States each year.’ With approximately 0.7% of the general U.S. population so affected, it has been estimated that personnel in an operatory treating 20 patients per day will encounter one active carrier in every 7 working days.* In addition to the problem of hepatitis transmission, the number of AIDS patients has been doubling approximately every 6 months.3 Effective sterilizing techniques are therefore needed for instruments, prostheses, and impressions contaminated with blood and saliva. Although instruments and prostheses can be effectively sterilized without harm, many types of impressions present a special problem because of their instability. One in vitro test has established that significant levels of organisms exist on dental impressions and gypsum casts and that bacteria survive on alginate surfaces for at least 1 hour.4-” Attempts to sterilize impressions have included the use of ethylene oxide gas, Sagrodent (Schiilke U. Mayr, Norderstedt, W. Germany), ultraviolet rays, sodium hypochlorite, iodophor, and a solution of 0.5% chlorhexidine in 70% alcohol43 ‘-‘I The American Dental Association (ADA) and the Centers for Disease Control now recommend a lo-hour soak in 2% activated glutaraldehyde solution as the most effective sterilization method, suitable for most impression materials except polyethers.‘,12 The first objective of this study was to measure and compare the linear dimensional changes of five representative rubber elastomers after their immersion in a 2% activated glutaraldehyde solution. A second objective was to test the effect on impression-acrylic resin assemblies to determine the appropriateness of sterilizing by immersion when acrylic resin trays are used in clinical practice.