Recombinant human interferon-beta (rhIFN-β) is the leading therapeutic intervention shown to change the cause of relapsing remitting multiple sclerosis and both a non-glycosylated and a significantly more active glycosylated variant of rhIFN-β are used in treatment. This study investigates the function of the rhIFN-β1a glycan moiety and its individual carbohydrate residues using the myxovirus resistance (Mx) mRNA as a biomarker in Mx-congenic mice. We showed that the Mx mRNA level in blood leukocytes peaked three hours after subcutaneous administration of rhIFN-β1a. In addition, a clear dose-response relationship was confirmed and the Mx response was shown to be receptor mediated. Using specific glycosidases, different glycosylation analogs of rhIFN-β1a were obtained and their activities determined. The glycosylated rhIFN-β1a showed significantly higher activity than its deglycosylated counterpart, due to a protein stabilization/solubilization effect of the glycan. Interestingly, the terminating sialic acids were essential for these effects. Conclusively, the structure/bioactivity relationship of rhIFN-β1a was determined in vivo and provided a novel insight into the role of the rhIFN-β1a glycan and its carbohydrate residues. The possibilities of improving the pharmacological properties of rhIFN-β1a using glycoengineering are discussed.