The in vitro hemolytic and in vivo mucosal irritation potential of ethylene oxide (EO) was investigated with standard procedures used to determine the biocompatibility of medical devices. Test solutions containing EO at concentrations of 25, 50, 100, 250, 500, 1,250, 2,500, 5,000, or 10,000 microg/mL were prepared in saline to simulate a worst-case aqueous extraction of standard medical devices containing 125, 250, 500, 1,250, 2,500, 6,250, 12,500, 25,000, or 50,000 microg/g of EO, respectively. Concentrations of EO up to 500 microg/mL were not hemolytic ( < 5% hemolysis after a 4-h exposure), whereas > or =1250 microg/mL of EO resulted in significant hemolysis. Hamster cheek pouches exposed to cotton pellets saturated with EO at concentrations of up to 2500 microg/mL for 4 h with a recovery period of 14 days were without effects attributable to EO. However, at > or =5000 microg/mL of EO, significant histomorphological alterations of the buccal mucosa were observed and attributed to EO exposure. It was concluded that solutions of EO of up to 500 microg/mL representing an aqueous extract of a general medical device containing at least 2500 microg/g of EO residue do not result in significant hemolysis and irritation.