Intravenous infusion of an analogue (f-met-leu-phe [FMLP]) of a bacterial-derived polymorphonuclear leukocyte (PMNL) chemotactic factor, or of the complementderived chemotactic stimulus, zymosan-activated plasma (ZAP, containing C5ad,, into rabbits induces acute PMNL margination in the pulmonary vasculature. This process also occurs during hemodialysis and the adult respiratory distress syndrome. The pulmonary PMNL sequestration is accompanied by thrombocytopenia. Because of the role platelets and PMNLs play in hemostasis and defense against infection, we studied the fate of these blood elements following sequestration induced by chemotactic factors. By employing "'In-labelled platelets and external radioisotope scanning, platelets were found to sequester in the pulmonary vasculature during FMLP infusion. Simultaneous 5'Cr PMNL and ' ' 'In-platelet studies showed that following sequestration, PMNLs returned to the circulation and disappeared with a normal half-life (T112) whereas the T1/2 of the platelets was markedly shortened (TI12 of control = 49 + 3.0 hr; FMLP or ZAP infused TI12 = 27 + 2.7 hr). Infusion of platelet-activating factor (PAF) induced PMN and ilatelet scc uestration with belled platelets showed that platelets did not release serotonin during FMLP, ZAP, or low dose PAF-induced sequestration. I n contrast to platelet survival, platelet size, platelet aggregation responses, and platelet glycoprotcins wcre not affected by transicnt sequestration. These resulls indicatc that during PMNL margination induced by relatively "pure" PMNL stimuli such as FMLP, platclcts may reversibly marginate and subsequently be cleared at an accelerated rate. The reason for accelerated platelet clearance is not ; I result of circulating pliitelet aggregates or detcctahlc proteolytic modification ol'membranc glycoproleins. Such altcrcd platelet kinetics may contribute to thromhocytopenia during sepsis, the adult respiratory distress syndrome, and other states in which excess I'MNI, tiiargination occurs. similar abnornmalitics in platelet kinetics. Studies with . $1 CI . -and "C-serotoniii-la-