The excretion of bile acids in urine was followed prospectively during the first year of life in 17 infants fed different diets from the age of 3 to 10 days. Eight infants were breast-fed, four were fed formulas that were based on adapted cow's milk, and five were fed a formula that was based on soy protein isolate. The formulas had higher protein concentrations than human milk; had different types of proteins, and had not been supplemented with taurine. Urinary bile acids were determined by gas-liquid chromatographic/mass spectrometric analyses of 24-h urinary samples collected at 1-12 days (only formula-fed infants) and at 1, 3, 6, 9, and 12 months of age. The results showed a higher urinary bile acid excretion at 3 months of age in both formula groups than in the breast-fed infants. A deficiency of dietary taurine during formula-feeding did not seem to limit the formation of taurine conjugates during the first month of life. The developmental pattern of urinary bile acid excretion during the first year differed according to the type of feeding. Isomers of cholic and chenodeoxycholic acid appeared in the urine of all breast-fed infants at 6 to 12 months of age. These metabolites, assumed to be the first metabolites derived from the developing gut flora of the infants, appeared at an earlier age and in higher amounts in both formula groups compared to breast-fed infants. Bile acids lacking a 7-hydroxy group, known to be formed by the intestinal flora, appeared in infants in all feeding groups later than the isomers. The results of the study imply that the early introduction of formula may modify bile acid metabolism in infants.