Investigation of the clotting factors in five humans accidentally exposed to total body irradiation disclosed no deficiencies. There was no evidence of a circulating anticoagulant in any case. Moderate fibrinolysis was seen in two instances in the same individual but was not evident in the others. At the eleventh day there was uniformly a moderate increase in the levels of fibrinogen. At the twenty-ninth day there was observed an increase in the levels of factors V and VIII coincidental with thrombocytopenia. This change reversed itself with recovery. A similar increase in the levels of factor VIII and less so of factor V was seen in cases of idiopathic thrombocytopenic purpura and secondary thrombocytopenia. An increase in level of factor VIII was observed in patients on oral coumarin anticoagulation. It is concluded that the supernormal levels of factors V and VIII are secondary to lowered thrombin levels in the circulating plasma as a result of the thrombocytopenia and not due directly to the effect of irradiation.