Purpose: Interactions between the chemokine receptor CXCR4 and its ligand stromal derived factor-1 regulate hematopoietic stem-cell trafficking. AMD3100 is a CXCR4 antagonist that induces rapid mobilization of CD34+ cells in healthy volunteers. We performed a phase I study assessing the safety and clinical effects of AMD3100 in patients with multiple myeloma (MM) and non-Hodgkin's lymphoma (NHL). Patients and Methods: Thirteen patients (MM, n = 7; NHL, n = 6) received AMD3100 at a dose of either 160 μg/kg (n = 6) or 240 μg/kg (n = 7). WBC and peripheral blood (PB) CD34+ cell counts were analyzed at 4 and 6 hours following injection. Results: AMD3100 caused a rapid and statistically significant increase in the total WBC and PB CD34+ counts at both 4 and 6 hours following a single injection. The absolute CD34+ cell count increased from a baseline of 2.6 ± 0.7/μL (mean ± SE) to 15.6 ± 3.9/mu;L and 16.2 ± 4.3/μL at 4 hours (P = .002) and 6 hours after injection (P = .003), respectively. The absolute CD34+ cell counts observed at 4 and 6 hours following AMD31OO were higher in the 240 μg/kg group (19.3 ± 6.9/μL and 20.4 ± 7.6/μL, respectively) compared with the 160μg/kg group (11.3 ± 2.7/μL and 11.3 ± 2.5/μL, respectively). The drug was well tolerated and only grade 1 toxicities were encountered. Conclusion AMD3100 appears to be a safe and effective agent for the rapid mobilization of CD34+ cells in patients who have received prior chemotherapy. Further studies in combination with granulocyte colony-stimuating factor in patients with lymphoid malignancies are warranted.