Polyethylene Glycol (PEG) is a long chain polymer that has been approved by the Food and Drug Administration for human intravenous, oral and dermal applications. Covalent attachment of PEG (PEGylation) to proteins can reduce their immunogenicity, minimize proteolytic cleavage and increase their serum half-life. PEG has also been attached to small molecules and liposomes for more selective delivery. PEG-modification of superparamagnetic iron oxide and quantum dots can improve their biocompatibility and reduce non-specific uptake. This video shows the procedure of sandwich ELISA assay for anti-PEG antibody pair.