Although antibodies to the lipopolysaccharide antigens of Shigella have been demonstrated in human milk, such antibodies do not explain the putative protective effect of breast-feeding against symptomatic Shigella infection. Shigella species do not share related lipopolysaccharides, but they do possess closely related virulence plasmids that code for the proteins essential for cell invasion. We therefore sought to determine the frequency, amount, and duration of excretion of human milk antibodies to these shared virulence plasmid-associated antigens in populations of different rates of Shigella infection frequency (Mexico City, high; Houston, low). Such antibodies were present in the milk of virtually all the Mexican women but also were present in a large proportion of milk samples from the women living in Houston. The amounts of these antibodies were highest in colostrum but after 2 weeks of lactation fell to stable levels. The frequency and persistence of these antibodies in the milk of the women from Houston suggest that the memory and drive for secretion of these antibodies is extremely long lived.