One of the most fascinating tools at the disposal of the molecular biologist is the medical clinic. The responsibilities of those who provide health care do not stop when they give optimal care to the individual patient and train their successors adequately. They also are under the obligation to obtain maximal information from every case they treat in order to reach a better understanding of the underlying illness in order to improve therapeutic results in the next patient. Fundamental research in pathological material is therefore a medical must as well as an opportunity for scientific work. The scientist working in this field can profit from nature's unasked for experiments, which are encountered by his medical colleagues in their clinical material. There are many examples of subjects of study - for instance hemoglobins and immunoglobulins - which started in a medical context and gradually developed into a field of prime interest for the molecular biologist. The study of blood coagulation is one of the younger areas of this kind.