[en] A method of obtaining a relatively purified splenic extract (the 1st fraction of KM-cellulose) with inhibitory-DNase I activity has been developed. It has been established that a single administration of the extract (the 1st fraction of KM-cellulose) 60 min. after γ-irradiation with LD of 80-90/30 increases survival by 50% as compared to that in the control animals, and produces a stimulating effect on the regeneration of marrow cellular tissue. A relatively purified splenic extract (the 1st fraction of KM-cellulose) with high inhibitory-Dnase I activity increases survival of irradiated animals by 50% as compared to that in the control animals. A relatively purified splenic extract (the 1st fraction of KM-cellulose) without inhibitory-DNase activity does not increase survival. This method is suggestive of a possible role of DNase I natural inhibitors in postradiation regeneration