At least 20 different human proteins can fold abnormally resulting in the formation of pathological aggregates and several deadly degenerative diseases. Evidence also suggests that non-discase-associated proteins, under appropriate conditions, can aggregate in vitro to form amyloid fibrillar species. Numerous reports have shown that the interaction between cell membrane and amyloid proteins is of particular importance in the cytotoxic effects elicited by amyloid proteins. Despite the significant progress has been made, there are still large gaps in our knowledge of the disease mechanism(s) associated with this aforementioned interaction. In the current research, using a dual-wavelength fluorescence ratiometric method along with a voltage-sensitive dye, di-8-ANEPPS, we found that a decrease in intramembrane dipole potential was observed upon binding of amyloid proteins with phospholipids and this decrease became more dramatic when protein was in its aggregated form. Moreover, our data revealed that a correlation among the presence of cholesterol, the type of phospholipid, and the drop in dipole potential was evident. In comparison with the pure DPPC, the relative difference in dipole potential between fibrillar and freshly prepared samples. attenuated with the addition of cholesterol while an increase in relative potential difference was observed in DPPG. Importantly, our results, for the first time, presented that the membrane dipole potential in amyloid protein-phospholipid interaction was dependent on the aggregation state of proteins, which is highly associated with the biological effects elicited by amyloid proteins. (c) 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.