Sulfadiazine (2-sulfanilamidopyrimidine), the pyrimidine analogue of sulfapyridine and sulfathiazole, is one of a group of heterocyclic derivatives of sulfanilamide synthesized by Roblin and his co-workers.1Laboratory studies on animals indicated that this drug has less toxicity than sulfapyridine and sulfathiazole and is highly effective against experimental infections with common pathogens.2Preliminary observations in this clinic3and in a number of others in which sulfadiazine was made available4indicated that this drug was worthy of extensive trial in the therapy of human bacterial infections. This paper deals mainly with a report of the clinical use of sulfadiazine in 446 adult patients treated at the Boston City Hospital prior to March 1, 1941. The results of laboratory studies will be reviewed briefly. ANIMAL EXPERIMENTS Feinstone and his associates2have shown that sulfadiazine is considerably less toxic than sulfapyridine or sulfathiazole both in acute experiments on mice and