Ethylene oxide is a known mutagen as indicated by short-term testing in vitro and in vivo. Occupational exposure can occur during ethylene oxide gas sterilization of materials for hospital and other use. To study the problem in a hospital sterilization facility where occupational exposure was suspected, epidemiologic, analytic and bioassay tools were employed. All persons whose work activities involved some aspect of the sterilization process were considered exposed to the gas. Within this group of symptomatic and asymptomatic individuals, chronic and incidental exposure was documented by clinical history. Sister chromatid exchanges were studied in lymphocytes cultured from exposed individuals as well as comparable controls. Four chronically exposed persons who reported upper respiratory and neurologic symptoms were studied in some detail. This group showed significantly increased sister chromatid exchange at three weeks and again at eight weeks after the last known exposure. Another group of eight persons with fewer complaints studied as late as the ninth week showed significantly increased exchanges. Incidental exposure may also increase sister chromatid exchange. The measured maximum concentration of ethylene oxide in the sterilizer room was 36 ppm (within standards set by the Occupational Safety and Health Administration).