SummaryTwo different investigations were conducted on the chemiluminescent activity of bovine milk polymorphonuclear neutrophil leucocytes (PMN) activated by different stimuli: zymosan, derived from the wall ofSaccharomyces cerevisiae(Experiment A), andStreptococcus uberisandEscherichia coli(Experiment B). In Experiment A, a quarter with a phagocytic 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 than a quarter with phagocytic activity above this threshold value. In Experiment B, calculation of the odds ratio showed similar results following stimulation withStr. uberisorEsch. coli. These results provide evidence that immunocompromisation of mammary gland defences could predispose to clinical mastitis. They also support the need to challenge phagocytic cells with appropriate stimuli, and theEsch. colitest seems to be the most sensitive.