Background: Colostrum is the first milk produced after birth and is rich in immunoglobulins and bioactive molecules. Objective: We examined if human colostrum and milk contained pancreatic secretory trypsin inhibitor, a peptide of potential relevance for mucosal defence and using in vitro and in vivo models, determined if its presence influenced gut integrity and repair. Designs: Human milk was collected from individuals over various times from parturition and PSTI concentrations determined using immunoassay. Human milk samples were analysed for proliferation & pro-migratory activity (wounded monolayers) and anti-apoptotic activity (caspase-3 activity) using intestinal HT29 cells +/- neutralizing antibodies to PSTI and EGF. Rats were restrained and given indomethacin to induce gastric injury. Effect of gavage with human breast milk +/- neutralizing antibodies on amount of injury were compared with animals receiving a commercial formula feed. Results. PSTI is secreted into human milk with colostrum containing a much higher concentration of PSTI than human milk obtained later. Human milk stimulated migration and proliferation about three-fold and reduced indomethacin-induced apoptosis by about 70-80%. Sixty-five percent of the migratory effect of human milk could be removed by immunoneutralisation of PSTI. PSTI worked synergistically with EGF in mediating these effects. Gastric damage in rats was reduced by about 75% in the presence of human milk and was more efficacious than the formula feed (p<0·001). Protective effects of the milk were reduced by about 60% by PSTI immunoneutralisation. Conclusions: PSTI is secreted into human milk at concentrations which have probable pathophysiological relevance.