Defibrinated sheep blood is used for the preparation of blood agar for the production of microorganisms and for the determination of hemolysis reactions.

Fibrins in the composition of the blood are one of the factors that coagulate the blood. The blood to be added to the medium must not be clotted. For this purpose, the blood is either mechanically separated from its fibrins or taken directly into bags containing anticoagulants. Fibrin, an insoluble protein that is produced in response to bleeding and is the major component of the blood clot. Fibrin is a tough protein substance that is arranged in long fibrous chains; it is formed from fibrinogen, a soluble protein that is produced by the liver and found in blood plasma.