The adhesive characteristics of hematopoietic stem and progenitor cells may partly regulate their proliferation and differentiation and may be critical in the homing of transplanted stem cells. Using quantitative adhesion assays, we have examined the characteristics of activated platelet adhesion to CD34+ cells in human blood and to the KG1a cell line. Approximately 85-95% of CD34+ cells from both sources bound thrombin-activated platelets; like mature neutrophils, activated platelet binding was maximal within 2 minutes of coincubation. Activated platelet adhesion to CD34+ cells was completely inhibited by chelation of calcium or by preincubation with the G1 blocking monoclonal antibody (MoAb) to platelet P-selectin. Using MoAbs to P-selectin glycoprotein ligand-1 (PSGL-1), we demonstrated that PSGL-1 was present on the surface of CD34+ cells; preincubation of CD34+ cells with the PL1 blocking MoAb to PSGL-1 completely inhibited activated platelet adhesion to CD34+ cells. Furthermore, treatment of CD34+ cells with O-sialoglycoprotein endopeptidase, which destroyed the PL1 epitope of PSGL-1, also abolished activated platelet-CD34+ cell binding. By contrast, MoAb directed against control epitopes of PSGL-1 or endopeptidase-sensitive epitopes of the CD34 molecule had no effect on activated platelet adhesion to CD34+ cells. Unlike mature neutrophils that, when activated, decrease P-selectin-dependent platelet adhesion because of redistribution of PSGL-1, phorbol ester treatment of CD34+ cells had no effect on their ability to bind activated platelets or PSGL-1 MoAbs. This study identifies PSGL-1 on CD34+ cells as the ligand for platelet P-selectin and suggests functional differences between mature and precursor hematopoietic cells in the regulation of surface PSGL-1 expression.