Studies of Escherichia coli O antigen specific antibodies in human milk, maternal serum and cord blood. Acta Paediatr Scand, 65:216, 1976.--The quantity and class specificity of E. coli O antibodies in human milk, maternal serum and cord blood was determined by the enzyme-linked immunosorbent assay. The predominant Ig-class of these antibodies in milk was IgA. The initially high levels of antibodies decreased 10-fold during the first days, but there seemed to be a fairly constant daily production of IgA antibodies during the first two months of the nursing period. There was no obvious decline in antibody content from morning to night or during a meal. The ratio milk antibody/serum antibody was very high for IgA, suggesting a local production. The ratios for the IgG were all less than 1, suggesting a restricted transfer from the serum. Ratios around 1 for IgM did not exclude some local production. In umbilical cord serum the amount of IgG E. coli O antibodies was higher than that in maternal serum. Small amounts of IgM and IgA antibodies was also demonstrated in some cases. In milk as well as in serum there were antibodies against a wide variety of E. coli O groups, not only to the actual E. coli strain dominating the mother's gut flora.