The terminal carbohydrate residues of the N-glycan on the immunoglobulin G (IgG) fragment crystallizable (Fc) determine whether IgG activates pro- or anti-inflammatory receptors. The IgG Fc alone becomes potently anti-inflammatory upon addition of α2–6-linked N-acetylneuraminic acid residues to the N-glycan, stimulating interest in use of this entity in novel therapies for autoimmune disease [Kaneko et al. (2006) Science313, 670–3]. Complete Fc sialylation has, however, been deemed challenging due to a combination of branch specificity and perceived protection by glycan–protein interactions. Here we report the preparation of high levels of disialylated Fc by using sufficient amounts of a highly active α2–6 sialyltransferase (ST6Gal1) preparation expressed in a transiently transformed human cell culture. Surprisingly, ST6Gal1 sialylated the two termini of the complex-type binantennary glycan in a manner remarkably similar to that observed for the free N-glycan, suggesting the Fc polypeptide does not greatly influence ST6Gal1 specificity. In addition, sialylation of either branch terminus does not appear to dramatically alter the motional behavior of the N-glycan as judged by solution NMR spectroscopy. Together these, data suggest the N-glycan occupies two distinct states: one with both glycan termini sequestered from enzymatic modification by an α1–6Man–branch interaction with the polypeptide surface and the other with both glycan termini exposed to the bulk solvent and free from glycan–polypeptide interactions. The results suggest new modes by which disialylated Fc can act as an anti-inflammatory effector.