Structural heterogeneity of recombinant IgG1 antibodies derives from variations in conserved as well as unique structural features. Common sources of heterogeneity include Fc glycosylation, partial heavy chain C-terminal Lys processing, Fc methionine oxidation, hinge-region cleavage, and the glycation of Lys residues. Aspartate residues that are isomerized to iso-aspartate were detected by cation exchange or hydrophobic interaction chromatography for trastuzumab and omalizumab, respectively. Unpaired cysteines were detected in omalizumab using Ellman's reagent, with the thiol-containing Fab resolved using hydrophobic interaction chromatography after papain digestion. Structural variations that cause chromatographic resolution may indicate the presence of a form with reduced potency.