The immune responses of 26 Angus-Hereford fetuses and neonates to E. coli O26:K60:NM were studied after bacterin or saline solution was injected (in utero) into the amniotic fluid. Calves were euthanatized at birth or were orally revaccinated; some were challenge exposed with live organisms. The hemolytic plaque assay was used to determine the presence of cells producing immunoglobulin[Ig]M, IgG1 and IgG2 in 4 segments of the small intestine, mesenteric lymph nodes and spleen. The passive hemagglutinin activity of intestinal washings was also determined. Anti-O26 passive hemagglutinin activity in the intestinal washings of principal calves was greater than in that of control calves, but in a given segment of the small intestine, this activity was usually relatively small and less consistent than the plaque-forming response. Greater numbers of plaque-forming cells were observed in the small intestine of 14 of the 15 principal calves when compared with the control calves tested.