In recent years PBPC have replaced bone marrow as the source of hematopoietic stem cells for autologous transplantation. One of the cited advantages of this procedure is the avoidance of bone marrow harvest, which frequently requires general anesthesia. Other advantages include faster neutrophil and platelet engraftment times, faster immune recovery, decrease in the amount of tumor contamination and technical ability to obtain stem cells from patients previously considered unharvestable because of marrow fibrosis or because of prior radiotherapy to the pelvis. Filgrastim has emerged as the preferred cytokine for stem cell mobilization based on its safety profile and the positive experience in granulocyte donors however, the number of circulating CD34+ cells does not occur until the third day after starting filgrastim injections. Pegfilgrastim stimulates the production and maturation of neutrophil precursors and enhances the functions of mature neutrophils in the same manner as filgrastim. Data form normal volunteers and in studies of patients with cancer have shown prolonged serum levels of the cytokine, with "self-regulation" of pegfilgrastim levels as a function of the neutrophil count. This confers a therapeutic advantage in clinical settings by allowing a less frequent dosing.