beta-Amyloid peptide (A beta) is the major constituent of senile plaques, the key pathological feature of Alzheimer's disease. A beta is physiologically produced as a soluble form, but aggregation of A beta monomers into oligomers/fibrils causes neurotoxic change of the peptide. In nature, many microorganisms accumulate small molecule chaperones (SMCs) under stressful conditions to prevent the misfolding/denaturation of proteins and to maintain their stability. Hence, it is conceivable that SMCs such as ectoine and hydroxyectoine could be potential inhibitors against the aggregate formation of Alzheimer's A beta, which has not been studied to date. The current work shows the effectiveness of ectoine and hydroxyectoine on the inhibition of A beta 42 aggregation and toxicity to human neuroblastoma cells. The characterization tools used for this study include thioflavin-T induced fluorescence, atomic force microscopy and cell viability assay. Considering that ectoine and hydroxyectoine are not toxic to cellular environment even at concentrations as high as 100 mM, the results may suggest a basis for the development of ectoines as potential inhibitors associated with neurodegenerative diseases. (c) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.