In investigations with mice the influence of an X-irradiation on the chemiluminescence yield of granulocytes from the peripheral blood was measured. The measurements were carried out with diluted freshly drawn blood by adding luminol as chemiluminogenic substance. The granulocytes were stimulated by zymosan. The following dependencies were investigated: dose from 1 to 8 Gy and time twelve hours to 32 days after a whole-body irradiation. In general the chemiluminescence yields from blood of irradiated animals are reduced in dependence of dose compared with the findings in controls. In unirradiated animals (controls) there exists a good correlation between the numbers of granulocytes and the chemiluminescence. This correlation is diminished after an irradiation. For the fourth day after a whole-body irradiation the chemiluminescence shows an exponential dose dependence. If chemiluminescence counts are related to the number of granulocytes (relative chemiluminescence) an increased chemiluminescence per single granulocyte is recorded up to the eighth day after an irradiation with 1 Gy. It may be assumed that subpopulations of granulocytes with enhanced ability to form oxidative metabolic products are responsible for this effect.