Mechanical treatment changes physically-mechanical characteristics of fibrin. The paper made from fibrin, which is not ground, has very bad mechanical characteristics. These apply also to papers, made of fibrin, which is only less ground. The degree of grinding grows parallelly to increasing time of grinding. At the same time, mechanical characteristics also grow to a certain maximum value, then they start to decrease. Curve of grinding growth, so as that of mechanical characteristics, depends on kind and qualities of fibrin. To define the quality of fibrin it must be subjected to laboratorial grinding using standard grind machines at standard conditions. Physically-mechanical characteristics of fibrin are to be defined with standard procedures as well. They both are prerequisite for the results gained by analysing the same or a different fibrin are mutually compared.