A method is developed for the quantitative determination of the protector activity. The method is based on registrating the coagulation time shortening for a mixture of fibrin monomer and fragment D. All determinations should be carried out at high concentrations of fragment D when its inhibitory effect on fibrin polymerization is very strong. It is necessary to control the exact conditions as to ionic strength and pH of the reaction mixture. Another approach to the protector activity estimation is demonstrated. It is based on determination of clot protein quantities after fibrin monomer clotting in mixtures with constant fragment D and different protector quantities.