SAS mice, treated intraperitoneally with 400 mg/kg of WR-2721 survived 9.5 Gy dose of X-irradiation. Prostacyclin release from lung tissue decreased in mice treated with WR-2721 and remained unchanched in mice treated with this drug and exposed to the lethal dose of X-rays. Thrombin activity in blood plasma of mice was not affected either by WR-2721 or X-irradiation. Also in mice radio protected and then irradiated this parameter remained unchanged. Fibrinogen level in blood plasma temporarily increased in mice treated with WR-2721 and significantly increased about a week after exposure of mice to the lethal dose of X-rays. This latter increase did not subside in mice which were treated with WR-2721 before irradiation.