Sulfisoxazole (gantrisin®), 3,4-dimethyl-5-sulfanilamido-isoxazole, introduced in 1946, is widely used in treatment of bacterial infections, especially those of the urinary tract. The drug is said to be one of the safest of the sulfonamides. It is described by the manufacturer as a "single drug—not a mixture of several sulfonamides— so that there is less likelihood of sensitization" and as "having greater solubility and higher blood-levels." As is the case with other members of the sulfonamide group, however, reports of toxic reactions have appeared in the literature. Aside from reports of nausea, vomiting, headache, fever, and dermatitis, a number of instances of toxic effects on the bone marrow have been reported. Haunz, Cardy, and Graham1collected eight cases of leukopenia following the use of gantrisin,®and recentlyHagerman and Franzblau2added another. A careful search of the literature for cases of hemorrhagic manifestations of toxicity has yielded one