The present invention relates to a method for in vitro determining generation of a haemostatis factor such as thrombin and/or plasmin in a test sample using a chemiluminescent substrate specific for said blood clotting factor. Upon cleavage of the substrate, a luminescent signal is generated via aminoluceferin with the aid of a luciferase. The invention also relates to a kit for in vitro determining generation of a haemostasis factor in a test sample, and to novel chemiluminescent substrates for the determination of thrombin and plasmin.