The invention discloses an immunoglobulin-binding protein comprising one or more mutated immunoglobulin-binding domains (monomers) of staphylococcal Protein A (E, D, A, B, C) or protein Z or a functional variant thereof, wherein in at least one of the one or more mutated monomers, the asparagine or histidine at the position corresponding to H18 of the B domain of Protein A or of Protein Z has been deleted or substituted with a first amino acid residue which is not proline or asparagine and wherein, if the amino acid residue at position 57 is proline and the amino acid residue at position 28 is asparagine, then the amino acid residue at the position corresponding to H18 of the B domain of protein A or of protein Z is not serine, threonine or lysine.