Abstract: The present invention presents an antigenic peptide whose amino acid sequence comprises a fragment of the caseinomacropeptide (CMP) sequence, which peptide is characterized in that it carries at least one epitope of CMP. These peptides exhibit no or little cross-reactivity with K-casein. When these peptides are used as immunogens, they make it possible to obtain specific anti-CMP antibodies. Also disclosed is a process for detecting the presence of cow's milk CMP in milk and milk products, using the above antibodies. Further presented are diagnostic reagents for the assay of CMP, which reagents are characterized in that they comprise at least one peptide, antigenic composition, or anti-CMP antibody of the present invention.