Adults and children 18 years of age and older who are donating stem cells for relatives enrolled in clinical trials at the National Institutes of Health may participate in this study. They will donate stem cells according to the standard procedure, but will give an extra 15 milliliters (3 teaspoons) of blood both before receiving G-CSF and after donating stem cells. Donors' spleen size will be measured by ultrasound scanning of the abdomen three times: the day before receiving G-CSF, the day after donating the stem cells and 4 days after donating.