This chapter reviews the reports on the mutagenicity of ethylene oxide (EO) with the aim of providing a critical evaluation of those studies most applicable to quantitative risk estimation in man. The data base for submammalian studies of the mutagenicity of EO is thought to be unsuitable for risk assessment in man. Experiments with cultured mammalian cells suggest that there is no threshold for the genotoxic action of EO. One method for measurement of DNA damage in the living animals is unscheduled DNA synthesis (UDS). UDS in germ cells of male mice was measured by Cumming and Michaud, and the most sensitive stages were determined to be mature spermatids and early sperm. Sister chromatid exchanges (SCEs) have become a popular method of studying primary DNA damage because of the ease and simplicity of scoring exchanges in cultured lymphocytes from animals and man. Chromosomal aberrations have been induced by EO in a human amniotic cell line in vitro.