The activation of human polymorphonuclear leucocytes (the respiratory burst) can be studied by measuring their chemiluminescent response. This technique was adapted to equine leucocytes to investigate the effects of cell number, activator concentration, enhancers of chemiluminescence, pH, temperature and inhibitors. Leucocytes were isolated from citrated blood from healthy horses and chemiluminescence was measured with a Bio-Orbit luminometer sensitive to 900 nm light. The optimal cell density for the maximal chemiluminescent response ranged from 10(6) to 10(7) leucocytes 600 microliters-1. Chemiluminescence increased as a function of temperature, and the concentrations of luminol, lucigenin and phorbol myristate acetate (PMA), and was pH related (optimal pH value = 8.0 for lucigenin and 8.5 for luminol). The inhibition of chemiluminescence by 5 x 10(-5) M azide was 88 per cent for luminol and 37 per cent for lucigenin. Superoxide dismutase (100 IU) totally inhibited the chemiluminescence response. Approximately 30 per cent variability in chemiluminescence was observed under the same assay conditions, depending on the origin of the leucocytes. Based on these results, the conditions selected for the measurement of equine leucocyte chemiluminescence were: 10(6) to 10(7) leucocytes 600 microliters-1, 1 x 10(-6)M PMA, 1 mM luminol or 0.4 mM lucigenin, physiological pH (7.4) and physiological temperature (37.8 degrees C). These conditions were similar to those used for measuring the chemiluminescent response of human leucocytes.