This chapter provides guidance and procedures used for the characterization of biotechnology-derived articles by isoelectric focusing. This chapter is harmonized with the corresponding chapters in JP and EP. Other characterization tests, also harmonized, are shown in the USP general information chapters Biotechnology-Derived Articles—Amino Acid Analysis 〈1052〉, Biotechnology-Derived Articles—Capillary Electrophoresis 〈1053〉, Biotechnology-Derived Articles—Peptide Mapping 〈1055〉, Biotechnology-Derived Articles—Polyacrylamide Gel Electrophoresis 〈1056〉, and Biotechnology-Derived Articles—Total Protein Assay 〈1057〉.