The present invention relates to methods to determine peptides presented on the surface of mammalian cells following addition to the cells of a protein. The present invention also relates to diagnostic tests based on the determination of such peptides or modified molecules resulting from determination of such peptides, such as pharmaceutical entities preferably having specific biological activity and reduced or enhanced immunogenicity when compared with the corresponding non-modified molecules. The methods according to this invention are preferably established with tools using mass spectroscopy (MS).