This study will determine whether magnesium replacement during apheresis can decrease side effects that donors commonly experience. Apheresis is a method of collecting large numbers of certain blood cells, such as white cells, stem cells, or platelets. In this procedure, whole blood is collected through a needle in an arm vein, similar to donating blood. The blood is separated into its components by centrifugation (spinning), the required cells are extracted, and the rest of the blood is returned to the body, either through the same needle or through another needle in the other arm.