The pathology of mixed injuries resulting from simultaneous action of several damaging factors on the organism is still insufficiently known. Peritonitis is the most frequent complication of injuries to the abdominal organs. Co-existence of peritonitis with radiation sickness impairs considerably the results of therapeutic management and prognosis. Surgical treatment is indicated in the latent period of radiation sickness or only in the period of recovery. In the case of diffuse peritonitis, the time of performing the operation is of essential importance for the prognosis. The purpose of the reported investigations was the study of the effect of ionizing radiation before exposure of the organism on the course of diffuse peritonitis and a trial of prolonging with an antibiotic the preliminary stage of the disease in which surgical treatment is effective. Investigations were carried out on 160 male Wistar rats weighing 250g on the average, divided into five groups. Group 1 served as control. In Group 2, the rats were only exposed to radiation.