The comparative ability of the complement anaphylatoxins C3a and C5a to mediate leukocyte adhesion and transendothelial migration in vivo and in vitro was investigated. Superfusion of IL-1β-stimulated rabbit mesentery with C3a resulted in a rapid and stable adhesion of rolling eosinophils, but not neutrophils, to postcapillary venules. However, C3a failed to evoke subsequent transmigration of the adherent eosinophils. In contrast, C5a induced both the rapid activation-dependent firm adhesion and transmigration of eosinophils and neutrophils through venular endothelium. C3a induced selective shedding of L-selectin and an increase in α M β 2 integrin expression on eosinophils but not neutrophils, while C5a induced shedding of L-selectin and up-regulation of α M β 2 integrin on both eosinophils and neutrophils. Both C3a-and C5a-dependent adhesion to venular endothelium was blocked by ex vivo treatment of eosinophils with anti-α 4 and anti-β 2 integrin mAbs. In vitro, both C3a (but not C3a desArg) and C5a (including C5a desArg)-dependent transmigration of eosinophils across IL-1β-stimulated endothelial monolayer was mediated by α 4 β 1 and α M β 2 integrins. Overall these studies suggest that C3a is eosinophil-specific chemotactic mediator that influences selectively eosinophil adhesion but not transmigration in vivo. C5a in contrast is a complete activator of integrin-dependent adhesion as well as transmigration of eosinophils and neutrophils.