The synthesis and preliminary biological assays of a new monofunctional reagent to reversible derivatize monoclonal antibodies is described. This compound, comprising a 1,4-polyiol moiety, is cleavable by means of sodium periodate in mild conditions; moreover it also contains a phenolic residue suitable for 125I labelling and a carboxylic group for reaction with epsilon-lysyl amino group of antibodies. These features are suitable to study the monoclonal antibodies cell-internalization process and antigen expression on cell surface. The 125I labelled reagent has been coupled to the monoclonal antibody AR-3, an IgG1 directed toward the CAR-3 antigen widely expressed on human ovarian and colorectal adenocarcinomas. The cleavage capability is tested in different conditions both on reagent and on derivatized MAb. Cell labelling experiments are performed both on target and untarget cell lines.