The invention relates to a conjugate exhibiting interferon .beta. (IFNB) activity and comprising at least one first non-polypeptide moiety covalently attached to an IFNB polypeptide, the amino acid sequence of which differs from that of wildtype human IFNB in at least one introduced and at least one removed amino acid residue comprising an attachment group for said first non-polypeptide moiety. The first non-polypeptide moiety is e.g. a polymer molecule or a sugar moiety. The conjugate finds particular use in therapy. The invention also relates to a glycosylated variant of a parent IFNB polypeptide comprising at least one in vivo glycosylation site, wherein an amino acid residue of said parent polypeptide located close to said glycosylation site has been modified to obtain the variant polypeptide having an increased glycosylation as compared to the glycosylation of the parent polypeptide.