Non-specific adsorption of serum proteins to Eupergit C (EC) and agarose during the process of immunoaffinity chromatography often leads to contamination of the specifically eluted antigens to be purified. This effect was studied by application of serum samples to a β-mercaptoethanol-blocked EC (EC-βME) column followed by analysis of proteins eluted with various elution buffers. Inclusion of polyethylene glycol (PEG 400 or 1500) in the loading buffer reduced the non-specific adsorption of proteins to EC but had an adverse effect on agarose. Covalent attachment of amino-PEG to EC and to epoxy-activated Sepharose mimicked the effect of PEG in solution with EC and resulted in a marked reduction in non-specific adsorption of serum proteins. Inclusion of PEG in the loading buffer during immunopurification of a serum protein (immunoglobulin G) or seminal plasma protein (human decidua protein hDP71) resulted in a marked improvement in the purity of these proteins eluted from the respective columns by ammonium acetate (pH 10).