polymerization properties of two normally circulating fibrinogens hmw and lmw evidence that the coohterminal end of the achain is of importance for fibrin polymerization the plasma fibrinogen fractions hmw mw 340000 and lmw mw 305000 were prepared from purified betaalanine precipitated fibrinogen by stepwise precipitation with ammonium sulfate the thrombin clotting times were 14 and 20 respectively the enzymatic phase of coagulation measured as release of fibrinopeptidea during incubation with thrombin was found to be identical for hmw and lmw polymerization was studied by light scattering at 605 nm using preformed monomers desaa and desaabb prepared from hmw and lmw in the presence of 33 m urea by incubation with thrombin 100 nih uml final conc and reptilase 1 uml final conc the hmwmonomers polymerised at a substantially higher rate than the corresponding lmwmonomers thus the prolonged clotting time of lmw was explained by retarded polymerization it is suggested that the cooh terminal end of the achain containing the molecular difference between hmw and lmw is of importance for polymerization furthermore the release of fibrinopeptide b desaabbmonomers improved polymerization properties in hmw as well as in lmw and all types of monomers polymerised more rapidly in the presence of ca