We have performed a detailed characterization of recombinant human growth hormone that included the identification of the entire sequence with disulfide linkages as well as subtle modifications by a sensitive liquid chromatography coupled online with tandem mass spectrometry (LC-MS) approach using the accurate peptide mass (FTICR MS) and sequence assignment (MS/MS measurement). The extent of oxidation, deamidation, and chain cleavages were measured by the ratio of peak areas of the nonmodified peptide vs. the sun? of peak area of the nonniodified and modified peptides in the same LC-MS analysis. The subtle but distinct differences were found in the recombinant human growth from the three manufacturers (the follow-on, counterfeit, and the original innovator products). In relative comparison, the follow-on product had the highest degree of oxidation at methionine residues, followed by the counterfeit product, and the original innovator product had the least amount of oxidation at all three sites with the similar oxidation order. In cases, the oxidation order was Met14 > Met125 > Met170. In contrast, the follow-on had the least amount of deamidation at aspargine (Asn149), and the counterfeit had the highest degree of deamidation at this site. For the chain cleavage, the follow-on product had the highest cleavage occurring at T 10 peptide (between Asn99 and Set-100), the counterfeit had the highest cleavage on T4 peptide, (between Glu30 and Phe31), and the original innovator product with the least amount of cleavages on both sites. These subtle but distinct differences are likely because of nonidentical manufacturing, formulation procedures, and storage conditions. 0 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 207-218, 2009