Ethylene is converted to ethylene oxide by cell-free preparations from mature cotyledons of Vicia faba L. cv. Aquadulce. The protein reactive agents dithioerythritol, dithiothreitol and p-chloromercuribenzoate inhibit enzyme activity. Physiologically active structural analogues of ethylene competitively inhibit its metabolism by cell free extracts and are metabolised by intact cotyledons.