The invention discloses a polypeptide with improved alkaline stability, which polypeptide comprises a mutant of a B or C domain of Staphylococcus Protein A (SpA), as specified by SEQ ID NO 1 or SEQ ID NO 2, or of Protein Z, as specified by SEQ ID NO 3, comprising at least the mutation wherein the glutamine residue at position 9 has been mutated to a tryptophan, leucine, glutamic acid, valine or lysine. The invention also discloses multimers of the polypeptide, as well as separation matrices comprising the multimers or polypeptides.