Human colostrum and a high molecular weight colostrum fraction (HMWF; > 14,000 D) prevented the adhesion of localized adherent (LA+) O111:H-enteropathogenic Escherichia coli (EPEC) to HeLa cells. This effect was abolished after absorption with an O111:H-LA + EPEC strain, but absorption with a LA- strain of same serotype had no effect on the process. A low molecular weight fraction (< 14,000 D), absorbed or not with LA+ or LA- bacterial strains, did not inhibit the adherence of E. coli to HeLa cells. IgA-depleted colostrum had no inhibitory effect on bacterial adhesion, demonstrating the critical role of this protein in the phenomenon. Heat inactivation of whole colostrum did not significantly modify the inhibition adherence levels. Immunoblots of O111:H-LA+ strain outer-membrane complex reacted with colostrum and HMWF showing that IgA antibodies were predominantly reactive with a 94-kD protein. These data confirm and extend observations about colostrum sIgA participation in adhesion inhibition of EPEC to HeLa cells and its response to a 94-kD outer-membrane protein.