The induction of sensitivity to various sulfonamide compounds and the possible physiologic mechanisms responsible are subjects of increasing interest and importance. This paper is a report of a reaction which we have observed in 12 patients, in whom a sensitivity to sulfathiazole was apparently induced by local application of it to diseased skin, and in whom the evidence of sensitivity appeared when the drug was later administered by mouth. These patients all suffered from various forms of eczematous dermatitis complicated by some degree of low grade secondary pyococcic infection and were treated for more than five days with 5 per cent sulfathiazole emulsion type ointment and later given sulfathiazole by mouth. A characteristic explosive type of systemic and dermatologic reaction occurred. These patients were part of a series of more than 1,000 patients suffering from various dermatoses whom we have treated with the sulfonamide compounds applied locally. The several reports