The aim of this study was to establish the effect of different concentrations of acetylsalicylic acid (ASA) on reactive oxygen species (ROS) production by phagocyting cells. A method of chemiluminescence level measurement was used. Acetylsalicylic acid depressed spontaneous and zymosan-induced chemiluminescence in whole blood samples and in secretion from inflamed udder samples in concentrations of 500 μg/ml and 100 μg/ml. ASA in a concentration of 50 μg/ml increased the chemiluminescence level in whole blood taken from healthy cows.