beta-Amyloid peptide (A beta), in fibrillar form, is the primary constituent of senile plaques, a defining feature of Alzheimer's disease. In solution assays, fibril formation exhibits a lag time, interpreted as a nucleation/ condensation-dependent process. The kinetics of fibrillogenesis is controlled by two key parameters: nucleation and elongation rate constants. We characterized the time course of A,beta fibril formation by measuring the scattering caused by peptide aggregates. We report here the interaction of A beta with three alkylammonium bromides (dodecyl, tetradecyl, and hexadecyl) at supra-and submicellar concentrations and their influence on the kinetic constants. We observed a dual behavior: surfactants promoted or retarded fibril formation in a concentration-dependent manner. Below a determined surfactant concentration (close to the corresponding critical micellar concentration in medium without peptide), surfactants favor aggregation, presumably by means of electrostatic interactions that destabilize the native conformation. Beyond such concentration, the stabilizing effects of the monomer predominate. As a general rule, surfactants delay but do not completely inhibit aggregation.