You will receive a test dose of busulfan by vein over about 60 minutes. This low-level test dose of busulfan is to check how the level of busulfan in your blood level changes over time. This information will be used to decide the next dose needed to reach the dose level that matches your body size. You will most likely receive this as an outpatient during the week before you are admitted to the hospital. If busulfan cannot be given to you as an outpatient, you will be admitted to the hospital on Day -11 (11 days before your stem cells are returned to your body) and the test dose will be given on Day -10.