Abstract: Compositions are presented that include a recombinant IL-11 PEGylated at equimolar to low molar excess of PEG to achieve a highly pure monoconjugate preparation, which provides improved half-life in serum while having desirable therapeutic activity and presenting fewer side-effects. Most preferably, the IL-11 is an N-terminally truncated human or humanized IL-11 and has a 20 Kd or 40 Kd branched PEG moiety, Y- or comb-shaped in particular, coupled to the N-terminal amino group. Such compounds are characterized by substantially increased stability in serum and sustained biological activity while exhibiting significantly reduced plasma expansion.