 Inclusion Criteria: - Diagnosis of ITP according to American Society of Hematology ASH guidelines at least 3 months before enrollment - Have completed at least 1 prior treatment for ITP - The mean of the 2 platelet counts taken during the screening and pre-treatment periods must be: less than 30 x 109L for those subjects not receiving any ITP therapy, with no count greater than 35 x 109L; less than 50 x 109L for those subjects receiving a constant dose schedule of corticosteroids, with no count greater than 55 x 109L - Hemoglobin greater than 10.0 gdL - Written informed consent Exclusion Criteria: - Any known history of bone marrow stem cell disorder - Any active malignancy. If prior history of cancer other than basal cell carcinoma or cervical carcinoma in situ, no treatment or active disease within 5 years before randomization - Unstable or uncontrolled disease or condition related to or impacting cardiac function e.g., unstable angina, congestive heart failure New York Heart Association NYHA greater than class II, uncontrolled hypertension diastolic greater than 100 mmHg or cardiac arrhythmia - Have 3 or more of the following predisposing factors for thromboembolic events: diabetes; smoker using oral contraceptives; hypercholesteremia greater than 240 mgdL; treatment for hypertension - Known positive test for human immunodeficiency virus HIV infection or hepatitis C virus - Received any treatment for ITP except for a constant dose schedule of corticosteroids within 4 weeks before the screening visit - Received IVIg or WinRho within 2 weeks before the screening visit - Received hematopoietic growth factors, including IL-11 Neumega within 4 weeks before the screening visit - Past or present participation in any study evaluating PEG-rHuMGDF, recombinant human thrombopoietin rHuTPO or related platelet product - Received any alkylating agents within 8 weeks before the screening visit or anticipated use during the time of the proposed study - Received any monoclonal antibody eg, rituximab within 16 weeks before the screening visit or anticipated use during the time of the proposed study - Less than 4 weeks since receipt of any therapeutic drug or device that is not FDA approved for any indication before the screening period - Less than 2 months since major surgery including laparoscopic splenectomy - Pregnant or breast feeding - Subjects of reproductive potential who are not using adequate contraceptive precautions, in the judgment of the investigator 