Amyloid plaques composed of proteinaceous aggregates are commonly found in brains affected by Alzheimer's disease and spongiform encephalopaties. A structural homology has been recently described for the Alzheimer's peptide A beta 1-28 and the segment of the prion protein Prp185-208. In the present paper, further elements in common are reported: the aggregation processes are in both cases enhanced by the model glucosaminoglycan heparin and dendrimers can modulate the aggregation process by affecting the nucleation rate at low concentrations and the elongation rate at high concentrations. Nucleation and elongation rate constants are derived from fittings to a nucleation dependent polymerization model. (c) 2005 Elsevier Inc. All rights reserved.