The present study has examined the effect of metals, such as iron and copper on the cytotoxicity of amyloid beta protein 1-40 (A beta 40). First, we showed that monomeric A beta 40 has stronger cytotoxicity than various type of aggregated A beta 40. Next we showed the addition of metals into the monomeric A beta 40 reduced the cytotoxicity of either monomeric A beta 40 or metals (iron and copper) although the addition of metals into monomeric A beta 40 resulted in a marked increase of aggregated form of A beta 40, which composed of beta-sheeted A beta 40 and A beta 40 aggregation not characterized by beta-sheet fibrils (coagrated A beta 40). Taken together, the metals and monomeric A beta 40 affect on each other and cause the reduction of their cell toxicity. (C) 2007 Elsevier Inc. All rights reserved.