Human milk samples from three healthy donors were investigated in order to evaluate the antibacterial activity during lactation against Escherichia coli ATCC 25922 and Listeria monocytogenes. The concentration of the main human-milk antimicrobial proteins (lactoferrin (LF), lysozyme (LZ) and secretory immunoglobulin A (sIgA)) was determined by ELISA. Results showed that human milk exhibited antibacterial activity against List. monocytogenes, although it was weakly active against Esch. coli ATCC 25922. The observed antilisterial activity was positively correlated with LZ concentration. In addition, the effect of gastrointestinal proteases, at different pH conditions, that prevail in the stomach of infants (pH 2.0-6.5), on antilisterial activity and protein degradation was evaluated. Hydrolysis with pepsin at pH 4.0-6.5, followed by treatment with pancreatic enzymes, resulted in a decreased hydrolysis of LZ, LF and sIgA and an enhanced antibacterial activity against List. monocytogenes. It is suggested that partial degradation of certain milk proteins at the gastrointestinal level may produce peptides that could act synergistically with the remnant intact proteins.