 The purposes of this study are: to determine a weekly dose that demonstrates a satisfactory safety profile and sustains elevated platelet counts within a targeted therapeutic level a doubling of baseline platelet counts and within the range of greater than or equal to 50 x 109L and less than or equal to 450 x 109L in thrombocytopenic subjects with ITP; to assess Amgen megakaryopoiesis protein 2 AMP2 AMG 531 pharmacokinetics PK in thrombocytopenic subjects with ITP. 