The Factor II, the prothrombine is a protein of the blood Coagulation.

The Thrombin (Factor II activated)) is produced by the cleavage of two sites prothrombine by the Factor Stuart activated (or Xa). It is about a serine protease () catalyzing the transformation of Fibrinogène into Fibrine. It also activates factors VII (extrinsic way), V and VIII (cofacteurs of the factors Xa and IXa respectively) and XIII.

It is about a factor Vitamine K dependant, the catch of Anti-vitamin K (AVK), the deficit in Vitamine K involves a deficit in factor II this translating by an increase in TCA and Temps of Quick (reduction in TP, increase in INR).