Here we report that aggregation of this COOH-terminal APP fragment A4CT and also of beta-A4 itself depends on additional factors. In contrast to the reticulocyte expression system, expression of A4CT and beta-A4 in the wheat germ expression system resulted in only monomeric forms. We have identified the factors which are capable of transforming both soluble A4CT and beta-A4 into insoluble and aggregating molecules. Monomeric A4CT or beta-A4 expressed in the wheat germ lysate could be transformed into aggregating molecules by the addition of metal-catalyzed oxidation systems. The addition of radical scavengers such as ascorbic acid, trolox, and amino acids prevented the aggregation process induced by the radical initiators. Thus, the aggregation of amyloidogenic APP fragments if analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis requires amino acid oxidation and protein cross-linking induced by radical generation systems.