Much attention has been given in recent years to the problems introduced by the massive transfusion of ACD preserved bank blood 1.The effect of cold when bank blood is given rapidly is one of several factors considered to have an adverse effect on the patient2-6 and various methods of warming the blood have been described to overcome this particular difficulty. The methods available consist of either warming the blood before administration commences71 8 or warming the blood during transfusion by using drip extension tubing and some form of water baths-17.Besseling et al. 8 in describing the specifications for any blood warming device stated that it should be simple, consistent, rapid, effective at high rates of transfusion and should have no deleterious effects on whole blood.The 'Hemokinetitherm' controlled fluid warmer 1 4 consists of a metal case containing an electric heating element and it holds a reservoir of 600ml of water (figure 1). It is thermostatically controlled to run at a maximum temperature of 40°C and there are two indicator lights showing when mains electricity is on and when the heating element is cycling. A dial thermometer indicates the temperature of the water bath. The reservoir takes four minutes to warm from room temperature to 37°C. A presterilised coil of vinyl tubing which is placed in the reservoir is twenty-five feet in length, has a capacity of 55ml and is connected between the giving set and the patient.Experiments were carried out to measure the effectiveness of the 'Hemokinetitherm' in warming blood at various rates of flow and to estimate some