Recombinant human interleukin-eleven (rhIL-11) is a pleiotropic cytokine which stimulates stem cell proliferation and affects multiple cell types. The protein has been demonstrated to provide clinical benefit in platelet restoration and Crohn’s disease. rhIL-11 is a highly basic protein that is rich in arginine, leucine and proline residues, lacks disulfide bonds, is not glycosylated and is not homologous with other known hematopoietic growth factors. In the development of stable dosage forms of rhIL-11, we have examined the protein’s stability toward methionine oxidation.