We compared the mechanisms of apolipoprotein E- (apoE-) and antioxidant- (AO-) mediated inhibition of beta-amyloid fibril (fA beta) formation in vitro, based on a nucleation-dependent polymerization model using fluorescence spectroscopy with thioflavin T. We first applied a kinetic plot to transform a sigmoidal time-course curve of fA beta formation from freshly prepared amyloid beta-peptides (A beta) into a straight line. Mathematical treatment of this plot demonstrated that the above-described sigmoidal curve is a logistic curve and provided us with a kinetic parameter t(1/2), the time when the rate of fA beta formation is maximum. t(1/2) of beta-amyloids (A beta) (1-42) and (1-40) were 18.7 +/- 1.7 min and 6.3 +/- 0.2 h, respectively (mean +/- SD, n = 3) and were independent of the initial A beta concentration examined. Although apoE extended t(1/2) of both A betas in a dose-dependent manner, AO did not. On the other hand, the final amount of fA beta formed was decreased by both apoE and AO dose-dependently. We then analyzed the effect of apoE and AO on the extension reaction of fA beta, based on a first-order kinetic model. Although apoE extended the time to proceed to equilibrium in a dose-dependent manner, AO did not. On the other hand, both apoE and AO dose-dependently decreased the final amount of fA beta formed. These results indicate that apoE and AO inhibit fA beta formation in vitro by different mechanisms and suggest the existence of multiple pharmacological targets for the prevention of fA beta formation.