Between Days -15 and -8, you will receive a low-level "test" dose of busulfan by vein over about 45 minutes to 1 hour. Test doses are used to study how your body breaks down busulfan and decide the dose of busulfan that you will receive. You may receive the test dose before Day -8 as an outpatient in the clinic, or on Day -8 as an inpatient in the hospital.