The increasing exposure of patients to an ever-growing list of drugs and chemicals has led to the definition of a variety of pathophysiologic mechanisms responsible for adverse drug reactions. A major stimulus for many of these studies has been the observation that the same drug administered in the same way and in the same dose to a large number of patients may be toxic to only a few. An important example of this type of phenomenon is the selective hemolytic action of sulfonamides. Soon after the introduction of sulfonamides as antibiotic agents, Harvey and Janeway1 noted the development of an acute hemolytic anemia in three patients receiving sulfanilamide for bacterial infections. One year later, Wood2 emphasized the frequency and selectivity of the phenomenon by observing that 21 of 522 patients treated with sulfanilamide had a hemolytic reaction. Wood also stated that "the majority of patients who have once