Histamine, a biogenic amine normally stored in basophilic granulocytes and tissue mast cells, can be liberated in pathological concentrations during inflammation, tissue damage, and hypoxemic cell injury. After the addition of aprotinin to banked blood, complete inhibition of histamine liberation from stored blood cells could be observed. In the present clinical trial, in which 40 consecutive patients underwent hysterectomy, a similar effect was achieved. Following administration of 2, 3.5, and 5 million KIU of aprotinin, the intraoperative increase of histamine concentration from 0.3 to up to 1.28 ng/ml could be almost completely inhibited. In addition, the aprotinin-treated patients showed higher numbers of intact thrombocytes and less disturbance of the clotting, fibrinolytic, and inhibitor systems compared to the control patients.