A method of isolating or purifying under denaturing conditions a protein that is fused with a) a peptide sequence of the formula Trp-Xaa-His-Pro-Gln-Phe-Yaa-Zaa (SEQ ID NO: 8) in which Xaa represents an arbitrary amino acid and Yaa and Zaa either both denote Gly or Yaa denotes Glu and Zaa denotes Arg or Lys, or b) with a peptide sequence that comprises a sequential arrangement of at least two streptavidin-binding modules, wherein the distance between the two modules is at least 0 and not greater than 50 amino acids, wherein one binding module has 3 to 8 amino acids and comprises at least the sequence -His-Pro-Baa-, where Baa is glutamine, asparagine or methionine, and wherein the other binding module has the sequence -Oaa-Xaa-His-Pro-Gln-Phe-Yaa-Zaa- (SEQ ID NO: 9) where Oaa is Trp, Lys or Arg, Xaa is any amino acid and where either Yaa and Zaa are both Gly or Yaa is Glu and Zaa is Lys or Arg, the method comprising contacting a sample containing the protein with a streptavidin mutein under suitable conditions to bind the peptide sequence to the streptavidin mutein, and separating the resulting complex from said sample, wherein the denaturing conditions are caused by the presence of guanidinium chloride in concentration of 2 M to 4 M, the presence of urea in a concentration of 1 M to 8 M, or the presence of hydroxide ions in a concentration of 0.05 M to 2 M, and wherein the streptavidin mutein (a) has a Cys residue at sequence position 127 with reference to the amino acid sequence of wild-type streptavidin as set forth in SEQ ID NO: 1, (b) wherein the sequence Val 44 -Thr 45 -Ala 46 -Arg 47 (SEQ ID NO: 10) or the sequence Ile 44 Gly 45 Ala 46 Arg 47 (SEQ ID NO: 11) is present in the region of amino acid positions 44 to 47 with reference to the amino acid sequence of wild-type streptavidin, and (c) comprises at sequence positions 117 to 121 with reference to the amino acid sequence of wild-type streptavidin an amino acid sequence selected from the group consisting of Glu 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 56), Asp 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 57), Glu 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 58), Asp 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 22) Gln 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 26), Asn 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 44) Asn1 17 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 66), and His 117 --- 118 --- 119 Trp 120 Tyr 121 (SEQ ID NO: 95). the method comprising contacting a sample containing the protein with a streptavidin mutein under suitable conditions to bind the peptide sequence to the streptavidin mutein, and separating the resulting complex from said sample, wherein the denaturing conditions are caused by the presence of guanidinium chloride in concentration of 2 M to 4 M, the presence of urea in a concentration of 1 M to 8 M, or the presence of hydroxide ions in a concentration of 0.05 M to 2 M, and wherein the streptavidin mutein (a) has a Cys residue at sequence position 127 with reference to the amino acid sequence of wild-type streptavidin as set forth in SEQ ID NO: 1, (b) wherein the sequence Val 44 -Thr 45 -Ala 46 -Arg 47 (SEQ ID NO: 10) or the sequence Ile 44 Gly 45 Ala 46 Arg 47 (SEQ ID NO: 11) is present in the region of amino acid positions 44 to 47 with reference to the amino acid sequence of wild-type streptavidin, and (c) comprises at sequence positions 117 to 121 with reference to the amino acid sequence of wild-type streptavidin an amino acid sequence selected from the group consisting of Glu 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 56), Asp 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 57), Glu 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 58), Asp 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 22) Gln 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 26), Asn 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 44) Asn1 17 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 66), and His 117 --- 118 --- 119 Trp 120 Tyr 121 (SEQ ID NO: 95). (a) has a Cys residue at sequence position 127 with reference to the amino acid sequence of wild-type streptavidin as set forth in SEQ ID NO: 1, (b) wherein the sequence Val 44 -Thr 45 -Ala 46 -Arg 47 (SEQ ID NO: 10) or the sequence Ile 44 Gly 45 Ala 46 Arg 47 (SEQ ID NO: 11) is present in the region of amino acid positions 44 to 47 with reference to the amino acid sequence of wild-type streptavidin, and (c) comprises at sequence positions 117 to 121 with reference to the amino acid sequence of wild-type streptavidin an amino acid sequence selected from the group consisting of Glu 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 56), Asp 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 57), Glu 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 58), Asp 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 22) Gln 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 26), Asn 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 44) Asn1 17 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 66), and His 117 --- 118 --- 119 Trp 120 Tyr 121 (SEQ ID NO: 95). Glu 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 56), Asp 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 57), Glu 117 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 58), Asp 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 22) Gln 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 26), Asn 117 Asn 118 Ala 119 Gly 120 Phe 121 (SEQ ID NO: 44) Asn1 17 Asn 118 Ala 119 Gly 120 Tyr 121 (SEQ ID NO: 66), and His 117 --- 118 --- 119 Trp 120 Tyr 121 (SEQ ID NO: 95).