Abstract: Process for the identification or assay of a particular polypeptide antigen X in which an immunological determination of the presence of said polypeptide antigen X is carried out in a sample, characterized in that the sample is treated in vitro using at least one chemical reagent reactingon an epsilon -NH.sub.2 function of a lysine residue oron a terminal alpha -NH.sub.2 function of a peptidein order to carry out the linking of said reagent to said polypeptide antigen X and in this way to obtain a modified polypeptide antigen X, (polypeptide antigen Xm), the immunological determination of the presence of said polypeptide antigen X in said sample is then carried out using at least one antibody produced by immunization against a polypeptide antigen Y identical to the polypeptide antigen Xm or to an antigenic fragment of this antigen Xm.