The invention discloses kappa light chain-binding polypeptide comprising a mutated binding domain of Peptostreptococcus protein L, wherein at least one asparagine residue of a parental domain defined by, or having at least 95% or 98% sequence homology with, SEQ ID NO: 2-6 or 2 has been mutated to another amino acid residue which is not asparagine, proline or cysteine.