With the growing use of sulfonamide drugs the problem of sensitivity to these drugs is taking on added importance. Besides the reactions which are known to occur one to two weeks following the institution of therapy there are also seen early reactions characterized by fever, chills, rash and pruritus within a few hours after the administration of these drugs and often in response to minute doses. Salvin1 in 1937 reported a case of such sensitivity to sulfanilamide. In 1939 Thompson2 described a similar response to sulfapyridine. And in 1940 Davidson and Bullowa3 noted fever with chilliness, erythema, pruritus and conjunctival injection some hours after the administration of sulfamethylthiazole. As far as I am aware, such reaction has not previously been reported with sulfadiazine, presumably because this drug is the most recent addition to the group. These reactions have been termed "hypersensitivity" or "acquired sensitivity" by most authors.