Ethylene oxide -- a widely used general-purpose fumigant of museum, library and archive materials -- is more toxic than previously supposed; it is now a recognized mutagen and is suspected of being a carcinogen. Aeration after fumigation is not so simple and rapid as formerly thought; tests have indicated that most materials will retain some ETO and release it slowly. Monitoring of personnel exposure, careful purging of the chamber and unloading procedures are among new precautions advocated. Investigation continues at the CCI.