Human colostrum and milk contain significant amounts of interleukin-1 receptor antagonist (IL-1RA) and both soluble receptors for TNFα (sTNF RI, sTNF RII). In a pilot study, we measured the urinary content of these components in 19 age-matched, breast-fed (BF)/formula-fed (FF) infants pairs 4- 184 days of age, to determine whether breast-feeding altered urinary content. Across all infant pairs, the concentrations of IL-1RA, sTNF RI and sTNF RII did not differ between BF and FF infants: BF IL-1RA>888±147 (mean±SE) pg/ml; FF IL-1RA >780±155 pg/ml; BF sTNF RI 1.3±0.3 ng/ml; FF sTNF RI 1.7±0.3 ng/ml; BF sTNF RII 4.0±1.0 ng/ml; FF sTNF RII 4.1±0.6 ng/ml. However, when infant pairs ≤56 days old (n=10) were compared, young BF infants had a higher urinary IL-1RA (>1037±215 pg/ml, p<0.008, paired t-test) than young FF infants (411±71 pg/ml), while urinary IL-1RA of older BF and FF infants were not different (BF >722±195 pg/ml; FF>1190±260 pg/ml, p=NS, paired t-test). Infant pair age did not influence urinary sTNF RI or sTNF RII in BF vs. FF infants. The ratios of sTNF RII to sTNF RI in urine of BF vs. FF infants were not significantly different(BF ratio 3.5±0.4; FF ratio 2.9±0.3), but the BF ratio was significantly higher than historically measured human milk ratios of RII to RI(milk ratio 1.9±0.4, p=0.03 vs BF ratio, t-test). These data suggest that breast-feeding enhances urinary excretion of IL-1RA in infants <8 weeks old, but cannot differentiate whether increased intake, increased endogenous production or both are responsible for the effect.