A batch fermenter modified to simulate the physical conditions of an inflamed v. uninflamed mammary gland was utilized to evaluate the effect of oxygen tension on killing of Escherichia coli P4 by bovine polymorphonuclear neutrophil leucocytes. Leucocytosis was simulated in vitro 4 h after inoculation with Escherichia coli by adding bovine neutrophils isolated from peripheral blood to the culture medium. Experiments were conducted at 39°C in UHT treated milk. At micro-aerophilic oxygen tension (oxygen partial pressure, 3·11kPa), bacterial numbers declined during the first hour following addition of the neutrophils. Oxygen tension declined rapidly following PMN addition. Once oxygen was depleted, neutrophil activity was presumably diminished and Esch. coli numbers began to increase. Under anaerobic conditions (oxygen partial pressure, 0·17kPa), no reduction in population was observed. Photomicrographs taken at the time of neutrophil addition and at subsequent time intervals demonstrated a specific association between neutrophils and the pathogen. Subsequent lysis of neutrophils associated with Esch. coli growth was seen coincident with oxygen depletion.