The present invention provides a fusion polypeptide comprising a first domain and a second domain wherein the first domain comprises a polypeptide ligand which binds to a cell surface associated antigen and the second domain comprises aglycosylated interferon a 2b (IFNa2b) having a sequence of SEQ ID NO: 1 or SEQ ID NO: 2. The aglycosylated IFNa2b further comprises one or more amino acid substitutions or deletions which attenuate the activity of the aglycosylated IFNa2b.