The protective effect of chicken egg yolk immunoglobulin Y (IgY) and its antigen‐binding FAb’ fragments was studied by their neutralization of heat‐labile toxin (LT) produced by enterotoxigenic Escherichia coli (ETEC) strain H10407. High levels of specific antibodies against LT were produced and maintained over a period of 8 months in eggs of hyper‐immunized hens. The FAb’ fragment produced by peptic digestion was found to be as effective as the whole IgY molecule in neutralizing LT. Removal of the enterotoxin‐specific antibodies eliminated the neutralization activities of both IgY and the FAb’ fragment. Heat pasteurization at 65°C for 15 min and presence of high salt (1.5 M‐NaCl) at pH 7.0 did not change the neutralization activities of either IgY or FAb’. Although incubation at pH of 2.0 for 4 h led to a 16‐fold decrease in the neutralization activities of IgY and FAb’, substantial neutralization activities were retained. The combination of low pH and high salt content, however, resulted in the complete...