Background: Plasma threonine concentrations are elevated in infants fed formula containing a whey-to-casein protein ratio of 60:40 compared with concentrations in infants fed formula containing a ratio of 20:80 or human milk (60:40). Objective: We studied whether degradation of excess threonine was lower in formula-fed infants than in infants fed their mothers' milk. Design: Threonine kinetics were examined in 17 preterm infants (gestational age: 31 ± 2 wk; birth weight: 1720 ± 330 g) by using an 18-h oral infusion of [1-13 C]threonine at a postnatal age of 21 ± 11 d and weight of 1971 ± 270 g. Five infants received breast milk. Formula-fed infants (n = 12) were randomly assigned to receive 1 of 3 formulas (5.3 g protein/MJ) that differed only in the whey-to-casein ratio (20:80, 40:60, and 60:40). Results: Threonine intake increased significantly in formula-fed infants with increasing whey content of the formula (48.5, 56.4, and 63.2 mol · kg Ϫ1 · h Ϫ1 , respectively; pooled SD: 2.2; P = 0.0001), as did plasma threonine concentrations (228, 344, and 419 mol/L, respectively; pooled SD: 75; P = 0.03). Despite a generous threonine intake by infants fed breast milk (58.0 ± 16.0 mol · kg Ϫ1 · h Ϫ1 ), plasma threonine concentrations remained low (208 ± 41 mol/L). Fecal threonine excretion and net threonine tissue gain, estimated by nitrogen balance, did not differ significantly among groups. Threonine oxidation did not differ significantly among formula-fed infants but was significantly lower in formula-fed infants fed than in infants fed breast milk (17.1% compared with 24.3% of threonine intake, respectively). Conclusion: Formula-fed infants have a lower capacity to oxidize threonine than do infants fed breast milk.Am J Clin Nutr 1999;69:105-14.