Plerixafor, given on day 4 of G-CSF treatment is more effective than G-CSF alone in mobilizing hematopoietic progenitor cells. We tested a strategy of preemptive plerixafor use following assessment of the peak mobilization response to 5 days of G-CSF. Patients were eligible for plerixafor if, on day 5 of G-CSF, there were <7 circulating CD34+ cells/μL or if <1.3 × 10(6) CD34+ cells/kg were collected on the first day of apheresis. Plerixafor (0.24 mg/kg s.c.) was given on day 5 of G-CSF followed by apheresis on day 6. This was repeated for up to two additional doses of plerixafor. The primary end point of the study was the percentage of patients who collected at least 2 × 10(6) CD34+ cells/kg. Twenty candidates for auto-SCT enrolled on the trial. The circulating CD34+ cell level increased a median of 3.1 fold (range 1-8 fold) after the first dose of plerixafor and a median of 1.2 fold (range 0.3-6.5 fold) after the second dose of plerixafor. In all, 15 out of 20 (75%) patients achieved the primary end point. In conclusion, the decision to administer plerixafor can be delayed until after the peak mobilization response to G-CSF has been fully assessed.