Systemic reactions following the intravenous administration of diodrast for excretory pyelography are frequently disturbing and occasionally serious. Many of the symptoms, such as sneezing, urticaria and wheezing, resemble allergic manifestations so closely as to suggest that allergy to the diodrast may be a cause for the reactions. On one occasion, after it was first determined that the intradermal injection of small amounts of diodrast was painless and caused no systemic reaction, increasing concentrations of diodrast were injected into the skin of a patient who had recovered from a severe reaction to diodrast given intravenously; there was a strong positive cutaneous reaction when 0.05 cc. of undiluted diodrast was injected intradermally. It was therefore decided to test in a similar manner all patients who were to receive diodrast intravenously for pyelography. The methods used and the results in a consecutive series of 404 patients are presented. METHODS On arrival in the