The apparent amount of factor VII as determined in a one-stage test depends on the type of thromboplastin used: bovine thromboplastin only reacts with human factor VIIa whereas human thromboplastin interacts with unactivated human factor VII as well. Therefore the ratio factor VII activity as measured with bovine thromboplastin divided by the factor VII activity as assessed with human thromboplastin reflects the state of activation of factor VII in plasma. This approach was used to study the process of cold promoted factor VII activation and the involvement of different clotting factors therein. It could be shown that cold promoted activation does not occur in the absence of factors II and XII and is reduced for about 50% in factor IX deficient plasma. The other coagulation factors have a minor influence on the process. The results indicate that the cold promoted factor VII activation is the result of activation by both activated contact products and thrombin.