Polyethylene glycol (PEG) is often conjugated with therapeutic proteins to enhance their PK properties. PEG may, however, be immunogenic, and the presence of PEG in food and cosmetics is believed to result in pre-existing anti-PEG antibodies in humans. Polyclonal and monoclonal antibodies reactive to PEG are provided for use in immunogenicity assay development to detect such anti-drug antibodies. Such antibodies exhibit preferential binding based on the size of PEG with molecular weight ranging from 350 daltons to 40 kD. Anti-PEG antibodies of the invention are engineered to comprise human Fc regions to enable non-bridging immunoassay formats.