The invention relates to an inhibitor of endogenous human interferon-gamma (hIFN-ϝ) in autoimmune diseases, especially in multiple sclerosis. More precisely, the invention relates to inactivated protein derivatives of the hIFN-ϝ with preserved affinity to the hIFN-ϝ receptor. The derivatives represent genetically modified variants of hIFN-ϝ where the C-terminal part of the molecule is either deleted or replaced with a polypeptide sequence of another human protein and a recombinant hIFN-ϝ inactivated by physical or chemical methods.