The interaction of activated platelets with leukocytes are believed to play an important role in ischemic reperfusion injury and other thrombotic conditions. Upon activation, platelets shed platelet microparticles (PMP) and express activation markers CD62P expressed on activated platelets mediates adhesion of platelets to leukocytes, chiefly neutrophils, but little is known of the interaction of PMP isolated from stored platelets or thrombin activated platelets was incubated with leukocytes and binding assessed by flow cytometry. FITC-labeled alpha-CD41 was used to assess platelet material associated with WBC. Like platelets PMP bound preferentially to neutrophils rather than lymphocytes, and exhibited an absolute dependence on the presence of Ca2+. Binding was time-and concentration-dependent, reaching a plateau at 10 min at a ratio of PMP to neutrophils of 150:1. Fluorescence microscopy showed that most of the neutrophils were aggregated into clusters of 5-20 cells. Clustering of neutrophils was not observed to result form interaction with platelets. In these clusters the adherent PMP appeared to serve as bridges between the neutrophil. Addition of EGTA after brief incubation (5-10 min) released most of the bound PMP but if added after > 10 min, only approximately 60% of bound PMP were released. In contrast, nearly all bound platelets were released by EGTA at the same time of incubation. Incubation of neutrophils with PMP gave significantly higher percentage of CD41a(+)neutrophils than did platelets incubated at the same numerical ratio. PMP association with neutrophils was less markedly inhibited by alpha-CD62P (AC1.2) than platelets, but binding of both PMP and activated platelets was inhibited approximately 90% by antisialyl Lewis X. PMP binding to neutrophils induced a significant increase in both CD11b expression and phagocytic activity in a concentration-dependent manner. These findings suggest a possible role for PMP in addition to providing platelet factor 3, specifically, as an activator and mediator of neutrophils in ischemic injury, thrombosis, and inflammation.