Two immunised and three unimmunised cows were challenged in a single mammary gland with 10(4) colony forming units of the vaccinal Escherichia coli strain. Immunisation comprised subcutaneous injection of killed bacteria with adjuvant at drying-off, and one intramammary infusion (without adjuvant) five weeks later. Somatic-cell counts and bacterial counts were monitored throughout the experiment. Bactericidal and opsonic properties of milk were assessed before inoculation and at 6, 12, and 24 h post-inoculation. Before challenge, cell-free milk of immunised cows enabled blood PMN leukocytes to kill the E. coli vaccine strain (B117) whereas in cell-free milk of unimmunised cows growth resulted. Nevertheless, in vivo E. coli B117 were able to grow in milk of all of the cows until they triggered an inflammatory reaction. Influx of cells started between 8 and 10 h post-inoculation in all of the cows but was more intense in immunised cows during the first six hours of inflammation. In vitro tests showed that whole mastitic milk acquired high bactericidal activity at the onset of inflammation. Although one immunised cow displayed clinical signs, milk yields of unimmunised animals were depressed to a higher extent. These results suggest that immunisation was able to enhance recruitment of phagocytic cells and to establish pre-inflammatory opsonic activity in milk.