Four cows were each vaccinated in two mammary glands with a live, formalin treated, Escherichia coli vaccine. Colostrum and milk samples taken in the first 3–4 days after calving, inhibited multiplication of the vaccine strain of bacteria. There was no bactericidal activity. After 4 days, multiplication was inhibited by milk whey from the vaccinated glands and not by whey from non-vaccinated glands. The difference in multiplication rate of E. coli added to whey from the vaccinated and non-vaccinated glands continued for at least 28 days. The antibody causing the multiplication inhibition was specific to the vaccine strain within the limits of serotypes tested.
Marked differences in indirect haemagglutinating antibody activity were found between mammary secretions from vaccinated and non-vaccinated glands and were still evident 464 days after vaccination.
Gnotobiotic pigs infected with one of the E. coli strains used for the preparation of vaccine were protected when fed whey from cows vaccinated with the homologous or heterologous antigens. Whey from non-vaccinated cows gave no protection.