Two different investigations were conducted on the chemiluminescent ac
tivity of bovine milk polymorphonuclear neutrophil leucocytes (PMN) ac
tivated by different stimuli: zymosan, derived from the wall of Saccha
romyces cerevisiae (Experiment A), and Streptococcus uberis and Escher
ichia coli (Experiment B). In Experiment A, a quarter with a phagocyti
c activity of PMN < 20 mV/1000 PMN following stimulation with zymosan
was found to be 23 times more likely to be a clinical mastitis case th
an a quarter with phagocytic activity above this threshold value. In E
xperiment B, calculation of the odds ratio showed similar results foll
owing stimulation with Str. uberis or Esch. coli. These results provid
e evidence that immunocompromisation of mammary gland defences could p
redispose to clinical mastitis. They also support the need to challeng
e phagocytic cells with appropriate stimuli, and the Esch. coli test s
eems to be the most sensitive.