Alpha-thrombin is a highly specific serine protease. During coagulation, thrombin cleaves fibrinogen to form fibrin leading to the formation of a fibrin clot. Thrombin is also responsible for feedback activation of the procofactors factor V and factor VIII. Thrombin has also been reported to activate factor VIII and platelets, and also functions as a vasoconstrictor protein.

Thrombin is supplied in 50% (vol/vol) glycerol/H₂O and should be stored at −20 ˚C. Purity is assessed by SDS PAGE analysis and activity is measured in a thrombin-specific clotting assay, and compared to standardised NIH thrombin.