SummaryThe disappearance of factor V-activity from human plasma on storage can be described as a first order reaction with an activation energy of about 44 kcal/mol.The disappearance of factor V- activity from bovine plasma in vitro is a second order reaction with an activation energy of about 66 kcal/mol.The inactivation of human prothrombinase during coagulation is a second order reaction; the activation energy is about 10 kcal/mol. It is concluded that this inactivation involves a reaction of the factor V moiety of prothrombinase with free factor V.