This paper demonstrates that human colostrum can inhibit the invasion of HEp-2 cells by enteroinvasive Escherichia coli (EIEC) of serotypes O28:H- and O29:H-, and that IgA antibodies mediate the inhibitory process. Seventy three of 77 (95.9%) colostrum samples prevented invasion of HEp-2 cells by E. coli O28:H-. Most of these samples contained high levels of IgA reactive to EIEC in immunoenzymatic assays. IgA eluted from an affinity chromatography column strongly inhibited HEp-2 invasion by EIEC, whereas IgA-depleted colostrum had no inhibitory effect on bacterial invasion. Immunoblots of colostrum samples with high (> 60%) invasion-inhibiting levels were performed with water extracts of invasive and noninvasive strains. Bacterial antigens from the invasive strain were recognized and the size of some was consistent with the invasion plasmid antigens (Ipas) A, B, C, and D, with stronger reactions with Ipas A and C. Colostrum samples with high inhibitory levels showed a strong reaction in Western blot assays, in contrast to the faint bands observed with poor-inhibitory samples. The results obtained in the present study suggest that colostrum IgA may protect infants against invasive E. coli infections.