While the procoagulant activity of platelet derived microparticles (PMP) has been widely accepted, knowledge regarding their immunological and adhesive qualities is still limited. It has been shown that murine BM cells covered with PMP engrafted lethally irradiated mice significantly faster than those not covered, indicating that PMPs play an important role in the homing of peripheral blood stem cells (PBSC). Here we studied the impact of PMP on engraftment in human allogeneic PBSC transplants for patients with hematological malignancies. PBSC samples were collected in buffered citrate from transplantation bags after infusion of transplants into patients with hematological malignancies (AML = 5, ALL = 1). Conditioning regimens included busulfan/cyclophosphamide (Bu/Cy), anti-CD66b-radioimmunotherapy (RIT)/Bu/Cy, and reduced intensity regimens with fludarabin/busulfan (Flu/Bu) and FLAMSA. Platelet-poor plasma (PPP) was prepared (1500g for 20min), immediately shock-frozen in liquid nitrogen and stored at −80°C. For further analysis PPP’s were carefully thawed at room temperature (RT). 90μl of PPP was stained with 5μl of CD41-PE and CD62P-FITC each for 15min at RT in the dark (IgG1-FITC and -PE served as negative controls, TRAP-6 (10μM) stimulated whole blood processed in same way as samples as positive control). To stop staining 900μl PBS/BSA 2% was added and 500μl of this solution were transferred into BD Trucount tubes by reverse pipetting giving a final concentration of 100 beads/μl. Samples were analyzed immediately using Coulter FC500 flow cytometer with CXP software. As expected the CD34 cell count (mean=5.1x106/kg body weight, SD=2.0x106/kg) showed a significant correlation (p=0.0197, Pearson r=−0.83) with the time to engraftment (mean=15.7days, SD=2.0d). The amount of CD62P positive microparticles (mean=423/μl, SD=119/μl) and the conditioning regimen showed no significant correlation with CD34 cell count or time to engraftment with leucocytes >1000/μl. In contrast, CD41-PMP count (mean=1223/μl, SD=857μl) correlated significantly with the CD34 cell count (p=0.0086, Pearson r=0.92) and the time to engraftment (p=0.0039, Pearson r = −0.95). Therefore, PBSCT contain significant amounts of PMP which are most likely generated during apheresis. Preliminary results show a stronger correlation with time to engraftment than does CD34 cell count. We conclude that PMP may accelerate engraftment of PBSC in humans. However, this function seems unrelated to P-Selectin expression. Therefore, further studies aiming to identify other adhesion molecules involved in PMP-mediated engraftment of PBSCT are warranted.