Forty-seven patients with stage II, III, or IV breast cancer undergoing autologous peripheral blood progenitor cell (PBPC) transplantation were randomized to placebo (n= 13) or to one of five sequential dose cohorts of pegylated (PEG) recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF)(1.0, 2.5, 5.0, 7.5, or 10.0 μg/kg/day)(n= 34). Blinded study drug was started on the day of transplantation and was continued until the platelet count was⩾ 100× 10 9/l or a maximum of 21 days. PBPCs were mobilized with filgrastim (r-metHuG-CSF) and all patients received filgrastim starting on day+ 2 after transplantation. The nadir platelet count was not affected by treatment. The median time to platelet recovery was 11 and 12 days for the placebo and combined PEG-rHuMGDF groups, respectively. No trends in adverse events suggested dose-or treatment-related toxicity. Two patients withdrew from the study because of an adverse event (allergic reaction in the 7.5 μg/kg group) probably related to study drug, and veno-occlusive disease (VOD)(in the 5 μg/kg group) which was felt not to be related to study drug by the investigator. No patients developed neutralizing antibodies to MGDF. Day+ 21 and day+ 28 platelet counts were higher in the group receiving PEG-rHuMGDF (246 vs 148× 10 9/l and 299 vs 145× 10 9/l, respectively; both P< 0.05). PEG-rHuMGDF up to 10 μg/kg/day was well tolerated. In this study, there was no effect of study drug on initial platelet engraftment at the doses studied. However, the efficacy of other doses is unknown. Bone Marrow Transplantation (2000) 26, 141–145.