A tracer-kinetic study using 1-13C- and 15N-labeled L-methionine was conducted in order to measure the retention rate of free methionine added to commercially-produced soy-based infant formulas. Twelve male infants, fed on a soy formula, received a single-pulse labeling by oral administration of L-1-13C-methionine (5 mg/kg) and L-15N-methionine (10 mg/kg). The abundance of expired 13C-labeled CO2 was measured up to 7 h after administration at 15-, 30-, and 60-min intervals. Additionally, enrichment of total 15N and 15N in urinary ammonia were determined up to 48 h after administration. Retention rates of the labeled carboxyl group amounted to an average of 91.2% (SD 4.1) of the intake. A similar retention rate was measured for the 15N-label of methionine (90.0%, SD 4.3). The data point at the efficacy of methionine supplementation of soy-based infant formulas.