We describe the characterization of an O-fucosyl modification to a serine residue on the light chain of a recombinant, human IgG1 molecule expressed in Chinese hamster ovary (CHO) cells. Cation exchange chromatography (CEX) and hydrophobic interaction chromatography (HIC) were used to isolate a Fab population which was 146 Da heavier than the expected mass. Isolated Fab fragments were treated with a reducing agent to facilitate mass spectrometric analysis of the reduced light chain (LC) and fragment difficult (Fd). An antibody light chain with a net addition of 146 Da was detected by mass spectrometric analysis of the modified Fab. A light chain tryptic peptide in complementarity determining region-1 (CDR-1) was subsequently identified with a net addition of 146 Da by a peptide map. Results from a nanospray infusion of the modified peptide into a linear ion trap mass spectrometer with electron transfer dissociation (ETD) functionality indicated that the modified residue was a serine at position 30 in the light chain. Acid hydrolysis of the modified tryptic peptide followed by fluorescent labeling with 2-aminoanthranilic acid (2AA) and HPLC comparison with monosaccharide standards confirmed the presence of fucose on the light chain peptide. The presence of O-fucose on an antibody has not been previously reported. Currently, O-fucose has been described as occurring on mammalian proteins with amino acid sequence motifs associated with epidermal growth factor (EGF)-like repeats or thrombospondin type 1 repeats (TSRs). The amino acid sequence around the modified Ser in the IgG1 molecule does not conform to any known O-fucosylation sequence motif and thus is the first description of this type of modification on a nonconsensus sequence in a mammalian protein.