This report describes a patient who experienced typical systemic allergic reactions associated with hemodialysis and the use of a plastic and rubber connecting tube in an arteriovenous shunt. A buffer solution placed in a connecting tube sterilized with ethylene oxide gas elicited positive reactions to skin tests. The patient had no detectable sensitivity to other substances considered to potentially represent alternative sources of the reactions. Human serum albumin exposed to ethylene oxide gas induced positive reactions in the patient and the release of histamine from his leukocytes. Corresponding tests in two control subjects were negative. Preliminary flushing of his hemodialysis system reduced the severity of reactions, apparently by removal of the sensitizing substance(s). It was concluded that there was sensitization and anaphylaxis from exposure to a product(s) of ethylene oxide gas used in sterilization.