We investigated the polymerization kinetics of murine senile amyloid fibrils (fASSAM) in vitro. When sonicated murine senile amyloid fibrils was incubated with its constituent monomer protein, the extension of amyloid fibrils was observed in an electron microscopic analysis. Quantitative fluorometric analysis with thioflavine T (Naiki H, Higuchi K, Hosokawa M, Takeda T: Anal Biochem 177:244, 1989) revealed that (a) extension of amyloid fibrils occurred by a pseudo-first-order exponential increase in the fluorescence of thioflavine T; (b) the rate of extension was maximal around pH 7.5, and was inhibited with the increase in KCl or NaCl concentration in the reaction mixture; (c) the rate of polymerization was proportional to the product of the murine senile amyloid fibrils number concentration and the constituent monomer protein concentration; (d) the net rate of extension was the sum of the rates of polymerization and depolymerization with the equilibrium association constant K of 5 x 10(7) M-1. These results show that amyloid fibril formation can apparently be explained by a first-order kinetic model: that is, extension of amyloid fibrils proceeds by consecutive association of precursor proteins onto the ends of existing fibrils.