In 1992, EPA's Atmospheric Research and Exposure Assessment Laboratory initiated a nationwide quality assurance (QA) program on the suppliers of EPA Protocol Gases. The program has three goals: to increase the acceptance and use of Protocol Gases by the air monitoring community, to provide a QA check for the suppliers of these gases, and to help the users of these gases to identify suppliers who can consistently provide accurately certified Protocol Gases. In this QA program which operates continuously, Protocol Gases are procured by EPA and the supplier's certification of the pollutant concentration(s) is verified by EPA. The results are published on the EPA Technology Transfer Network's electronic bulletin board. If a supplier's concentration differs from EPA's by more than 2%, the supplier is notified in writing immediately. The results obtained for SO2, CO and NO Protocol Gases are presented.