Acidification of the urine was studied with two groups of infants, each group consisting of four infants between 2 and 6 months of age: Group I received fresh or pasteurized human milk and Group II received whole cow's milk on evaporated milk and water without additional carbohydrate. The mean titratable acidity of the urine of infants of Group I was 3 meq/m2/12 hr compared with 35 meq/m2/12 hr for infants of Group II. The rates of excretion of ammonia were 7 and 28 meq/m2/12 hr by infants of Group I and II, respectively. After administration of ammonium chloride (4 gm/m2/day) for 2 days, the titratable acidity increased to 6 meq/m2/12 hr in Group I and to 34 meq/m2/12 hr in Group II. The mean rates of excretion of ammonia increased to 21 and 36 meq/m2/12 hr, respectively. The greater titratable acidity of the urine of infants of Group II is attributed primanly to the greater amounts of phosphate in the urine (mean, 35 mmol/m2/12 hr excreted by infants of Group II compared with 3 mmol/m2/12 hr by infants of Group I). The greater rate of excretion of ammonia by infants of Group II is attributed jointly to the prolonged administration of a diet with relatively great residue of anions and, perhaps, to the greater availability of glutamine and other precursors of ammonia.