The availability of monoclonal antibodies (mAbs) which bind to a specific antigen at distinct and well-defined sites led to a better understanding of the effect of highly specific antigen-antibody interactions on antigen behavior. We found that appropriate mAbs interact with their antigens at sites where unfolding is started, leading to stabilization, refolding and suppression of further aggregation. These effects were found to be related to the localization of the antigen site of the antibody. In the following we study the interaction of carboxypeptidase A (CPA) and Alzheimer's β-amyloid peptide (βAP) with some of their selected monoclonal antibodies. From a large panel of mAbs we selected two mAbs which exhibit significant chaperone-like activity in the refolding and prevention of CPA aggregation induced by heat and excess of Zn ions. CPA exposed to aggregation conditions maintains its solubility and catalytic properties in the presence of mAbs CP 10 and CP 9 . In vitro aggregation of βAP (1-40) was inhibited by the presence of one of its mAbs, called AMY-33, but not by mAb 6F/3D, reinforcing the dependence of prevention of aggregation on the localization of aggregating epitopes. Identification of sequences or regions that may play a role in the protein folding pathway via appropriate mAbs may lead to understanding and prevention of protein aggregation.