The purpose of this study was to quantify the growth promoting effect of a mixture of antibiotics for rats eating diets deficient in protein or an essential amino acid. Male albino weanling rats (70 to 80 g weight, 4 weeks old) were fed (a) a control diet containing all other required nutrients and varying amounts of casein (0 to 27%), or (b) a purified amino acid diet containing all other required nutrients and varying amounts of valine (0 to 70 mumoles/g diet), threonine (0 to 69 mumoles/g diet) or tryptophan (0 to 8.6 mumoles/g diet), with and without an oral antibiotic supplement consisting of neomycin sulfate (10 mg/100 g body weight/day), bacitracin (500 units/100 g body weight/day), and polymyxin B sulfate (1 mg/100 g body weight/day). At suboptimal intake of casein, valine, tryptophan or threonine, rats eating antibiotic-enriched diet showed up to 3 times greater daily body weight gain (deltaBW) than rats eating a similar diet without antibiotics. The growth-promoting effect of antibiotics can be expressed as percent sparing of specified nutrient (casein or individual amino acid), defined as below: (see journal) where nutrient intakeO ab or nutrient intakeab represents that intake of casein or of a particular amino acid which is required to produce a specific deltaBW in antibiotic-free or antibiotic-supplemented group, respectively. The percent sparing was inversely proportional to the dietary content of casein or limiting amino acid. For diets containing 10% to 25%, 25% to 50%, 50% to 75%, and 75% to 100% of the daily requirement of the limiting nitrogenous nutrient, sparing on the average was greater than 80%, 60%, 20%, and less than 10%, respectively. Data on daily food intake of ad libitum fed rats, and data from an experiment with tube-fed rats, showed that the growth-enhancing effect of antibiotics was independent of changes in food intake.