The expression on neutrophil-derived microparticles of the integrin Mac-1 (α_Mβ₂) in an active conformation provides a new mechanism for leukocyte-platelet crosstalk. In contrast to microparticles from resting neutrophils, microparticles from PAF- or PMA-stimulated neutrophils express Mac-1 where the insert or ‘I’ domain on the α_M subunit is in a high-affinity ligand-binding form. This can bind platelet GPIbα (the major ligand-binding subunit of the GPIb-IX-V complex), leading to platelet activation and P-selectin expression, which reinforces association via microparticle PSGL-1.