Mouse monoclonal immunoglobulin G2b (IgG2b) antibodies are known to contain two forms of the heavy chain that are different in susceptibility to the protease attack. In the present study, by use of an affinity column containing sialic acid-binding lectins from Maackia amurensis seeds, a mouse monoclonal IgG2b was successfully separated into three phenotypes, which are different in the degree of sialylation in the heavy chain. In the N-linked oligosaccharides from all of the IgG2b phenotypes, virtually no sialylation was detected. Elution profiles of the lysyl endopeptidase digestion products were compared for the three phenotypes. The peptides eluted at different retention times were subjected to fast atom bombardment-mass spectrometry and amino acid sequence analyses. It was revealed that approximately 40% of the heavy chain of the mouse IgG2b are O-glycosylated at Thr-221A in the hinge region, predominantly with a tetrasaccharide composed of GalNAc, Gal, and two N-glycolylneuraminic acid residues. We suggest that the O-glycosylation renders the hinge region resistant against the proteolyses of the heavy chain. A therapeutic significance of the O-glycosylation of IgG2b is briefly discussed.