Leptin, the recently cloned product of the obese (ob) gene, is a 16 kDa-protein that acts as a circulating satiety factor. It also serves to regulate energy expenditure and may act as a counter regulatory hormone to insulin. Initially thought to be exclusively produced by mature adipocytes, its mRNA has now been identified in significant levels in the placenta as well as the fetus raising speculation regarding its importance as a growth factor. Given studies demonstrating that exclusively breast-fed infants are leaner due to decreased energy intakes than formula-fed infants, we hypothesized that the presence of leptin in human milk could participate in mediating the earlier satiety of those infants fed human milk. We undertook this initial study to qualitatively examine the presence of leptin in human milk utilizing an immunoblot approach. Random milk samples during the first 2 weeks of lactation were available for study from 4 mothers delivering at term. Milk samples were centrifuged, the aqueous layer removed, and the protein content quantitated. One-hundred micrograms of total protein were separated by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis (SDS-PAGE), transferred to nitrocellulose, and immunoblotted with an antileptin antibody. As controls, recombinant human leptin alone and a sample of milk containing added leptin were similarly electrophoresed and immunoblotted. Labeled proteins were visualized by chemiluminescence. Significant amounts of leptin protein were identified in all milk samples examined. No difference in protein detection was identified in fresh milk vs. frozen milk, and little difference was apparent in foremilk samples vs. hindmilk samples. These preliminary data reveal the presence of leptin in term human milk and suggest that further studies to document bioactivity of milk-derived leptin are warranted.