The administration of granulocyte-colony stimulating factor (G-CSF) leads to splenic enlargement in approximately one third of neutropenic patients who are chronically treated with this drug. Short courses, 5 to 6-days, of G-CSF are used to increase the yield of hematopoietic progenitor cells collected by apheresis from peripheral blood stem cell (PBSC) donors. G-CSF mobilized PBSC concentrates are increasingly replacing marrow as a source of hematopoietic progenitor cells for transplants involving HLA-compatible relatives. PBSC donors experience splenomegaly and rarely, spontaneous rupture of the spleen. Since splenomegaly is a risk factor for splenic rupture, it is important to determine the incidence and time course of splenic enlargement in PBSC concentrate donors. This protocol will prospectively study 40 subjects donating G-CSF mobilized PBSC for HLA compatible relatives or donating PBSC's for laboratory investigations at the Department of Transfusion Medicine in the Warren G. Magnuson Clinical Center. The subjects will be given G-CSF for 5 days and a PBSC concentrate will be collected on day 5. Ultrasound scans will be used to assess their spleen size. The scans will be performed before G-CSF administration and the day the last PBSC concentrate is collected. A third scan will be performed 10 or 11 days after the last PBSC is collected. The effects of several parameters on spleen size will be assessed including: donor age, gender, race and changes in liver function assays, white blood cell counts, platelet counts, and CD34+ cell counts.