Copolymeric NiPAM:BAM nanoparticles of varying hydrophobicity were found to retard fibrillation of the Alzheimer's disease-associated amyloid beta protein (Abeta). We found that these nanoparticles affect mainly the nucleation step of Abeta fibrillation. The elongation step is largely unaffected by the particles, and once the Abeta is nucleated, the fibrillation process occurs with the same rate as in the absence of nanoparticles. The extension of the lag phase for fibrillation of Abeta is strongly dependent on both the amount and surface character of the nanoparticles. Surface plasmon resonance studies show that Abeta binds to the nanoparticles and provide rate and equilibrium constants for the interaction. Numerical analysis of the kinetic data for fibrillation suggests that binding of monomeric Abeta and prefibrillar oligomers to the nanoparticles prevents fibrillation. Moreover, we find that fibrillation of Abeta initiated in the absence of nanoparticles can be reversed by addition of nanoparticles up to a particular time point before mature fibrils appear.