The influence of sulfadiazine on the course of the more common infections of the respiratory tract has been studied in this laboratory since 1941.1 It will be recalled that originally 1 ment with sulfadiazine was started at the onset of symptoms of respiratory illness in order to establish whether bacteriostatic action on susceptible organisms primarily or secondarily involved might, in turn, affect the severity of the disease. While some of the patients observed during 1941 and 1942 were aided by the early use of sulfadiazine, others apparently were not benefited. The ameliorating effects observed were most striking in January and February 1942, during an outbreak of severe illnesses which were characterized by severe toxemia and high incidence of acute bronchitis and of pneumonia due to Pneumococcus type I. Shortly afterward sulfadiazine was used without measurable effect in an outbreak of relatively mild, uncomplicated infections presumably of viral origin.1a