A modified interferon beta (INFβ) is provided, which is less immunogenic than human INFβ (SEQ ID NO: 1) when administered to a human in vivo. The modified INFβ comprises an amino acid residue sequence that differs from SEQ ID NO: 1 by a substitution at one or more residues of SEQ ID NO: 1. Preferred substitutions are at residues selected from the group consisting of residue 50, 59, 60, 62, 63, 66, 67, 69, 70, 125, 126, 129, 130, 132, 133, and 138. Examples of suitable substitutions include F50A, L57A, I59A, Y60N, M62A, L63A, I66T, F67H, I69A, F70A, Y125A, Y126A, I129A, L130A, Y132S, L133A, Y138H, and Y138A.