The formation of amyloid-beta peptide (A beta) aggregates at an early stage during the self-assembly process is an important factor in the development of Alzheimer's disease. The toxic effect is believed to be exerted by prefibrillar species of A beta. It is therefore important to identify which prefibrillar species are toxic and characterize their distinct properties. In the present study, we investigated the in vitro aggregation behavior of A beta-derived peptides possessing different levels of neurotoxic activity, using fluorescence spectroscopy in combination with transmission electron microscopy. The toxicity of various A beta aggregates was assessed by using cultures of human neuroblastoma cells. Through combined use of the fluorescence probe 8-anilino-1-napthalenesulfonate (ANS) and the novel luminescent probe pentamer formyl thiophene acetic acid (p-FTAA), we were able to identify those A beta peptide-derived prefibrillar species which exhibited cellular toxicity. In particular, species, which formed early during the aggregation process and showed strong p-FTAA and ANS fluorescence, were the species that possessed toxic activities. Moreover, by manipulating the aggregation conditions, it was possible to change the capacity of the A beta peptide to form nontoxic versus toxic species. (C) 2012 Elsevier Inc. All rights reserved.