A significant reduction of mortality after lethal irradiation (7,8 and 9 Gy X-ray total-body irradiation) was achieved by continuous therapeutic subcutaneous application of the biologic inducers PIND-AVI and PIND-ORF. This was obtained by a stimulation of the investigated spleen parameters and a stimulation of leucocytes and phagocytosis. The inducers had no significant influence on the radiogenic reduction of blood cells and bone marrow cells, of the relative spleen and thymus weight and of the DNA and protein level of spleen and thymus. The regeneration of blood leucocytes (preponderantly by PIND-AVI) and of the spleen (preponderantly by PIND-ORF) was accelerated, but not the thymus regeneration which was already rather low. The leucocyte phagocytosis which increased generally after irradiation was markedly stimulated by paramunization. This stimulation was also observed after a latent time in animals submitted to sham irradiation. Its temporal appearance and its quantitative and qualitative properties corresponded to the effects of inducers after X-ray irradiation. Both inducers were found to be harmless in all experiments.