INTRODUCTIONTHE use of antibiotics in animal feeds has opened up a new field of investigation. The growth promoting effect of antibiotics has been observed in chickens, turkeys, pigs and rats. Responses vary with the species, the type of diet, level of antibiotic fed and stage of growth. The use of antibiotics in mixed feeds is now widespread, yet not all of their effects have been fully investigated.Previous work had shown that at least in some instances the use of an antibiotic in the feed of turkey poults resulted in increased body weight and more efficient utilization of feed, and seemed to reduce the need for the relatively more expensive animal protein products in the diet. However, much of the work had been done with numbers too small for proper statistical analysis, and few poults had been carried beyond 4 weeks of age. There had been popular reports that …