It is not possible to sterilise whole blood products and therefore they must be collected aseptically. Horse and sheep blood are the most widely used animal blood products in culture media. The choice of which type of blood to use with culture media is largely traditional, with much of continental Europe preferring sheep blood, whilst the UK and certain parts of the Commonwealth prefer horse blood. Defibrinated horse blood is aseptically collected whole horse blood that has been processed to remove fibrin. There are no additives or preservatives in this product. Defibrination is now accepted as the best method of preventing blood clotting. It must be carried out immediately after drawing the blood and the agitation must be sufficient to denature the fibrinogen but not to cause rupture of the erythrocytes and haemolysis. The haemolytic reactions of horse blood are not identical to sheep blood and blood agar media designed for horse blood may not be satisfactory with sheep blood and vice versa.