EPIDEMIOLOGY UNIT NUMBER 22 A previous paper has described the pronounced reduction of streptococcic disease which occurred at a large naval training center in the Northwest as the result of sulfadiazine prophylaxis.1 The present report deals with the loss of effectiveness of this prophylaxis as the result of the appearance of sulfadiazine resistant strains. On March 1, 1944 all personnel at the center were placed on sulfadiazine prophylaxis and no control studies were carried out. Shortly thereafter, during this month of March, there was a decided increase in streptococcic disease in Camp Hill, one of the recruit camps, which at the beginning of March had been on sulfadiazine prophylaxis for a period of approximately three months. Streptococcic infections were approximately four times as frequent as they were in January, when the drug had been most effective (table 1). Furthermore, during January only 22 per cent of admitted patients with respiratory