This study will determine whether the spleen in people who donate stem cells (bone marrow cells collected from the blood) enlarges as a result of taking granulocyte colony-stimulating factor (G-CSF). Donors take this growth factor to maximize the amount of stem cells that can be collected for transplantation to patients with leukemia or other diseases. The study will also examine whether changes in the donor's white blood cell count, blood stem cell count, and blood chemistries can predict a change in spleen size.