To the Editor:— Photosensitizing agents may reach the skin by contact, injection, or ingestion. Among those ingested are the following: sulfonamides and related drugs, the barbiturates, quinine, phenothiazines,Rauwolfiaalkaloids and several of the antibiotics. The purpose of this paper is to add a new photosensitizer to the above list. A 22-year-old white female, in general good health, presented with a history of recurrent urinary tract infections. In July, 1963, she complained of increased frequency of urination and discomfort on urination. Catheterized urine contained 20-30 white blood cells and 40-50 red blood cells per high-power field. Urine culture was negative. The patient took tetracycline, 250 mg four times daily for one week, and three weeks later was asymptomatic. In November, 1963, increased frequency and dysuria again developed, and again urine culture was negative, although there was an increased number of WBC in the urine. Treatment consisted of sulfisoxazole (Gantrisin), 4