Eosinophils play a pivotal role in the pathogenesis of asthma and allergic disease. The accumulation and persistence of eosinophils at sites of inflammation are mediated at least in part by the extended survival of eosinophils in response to circulating hematopoietins IL-3, IL-5 and GM-CSF. The apoptosis and subsequent clearance of eosinophils without histotoxic mediator release is thought to be crucial in the resolution of airway inflammation in asthma. This study characterised the morphological and biochemical events of human eosinophil apoptosis in vitro and investigated the mechanism by which IL-5 induces eosinophil survival. Peripheral blood eosinophils have a distinct expression profile of Bcl-2 homologues, critical regulators of apoptosis, with detectable expression of pro-apoptotic family homologues Bax, Bcl-xs, Bim, Bak, Bid, and Bad, and anti-apoptotic homologue Bcl-xL, with little or no detectable Bcl-2 expression. Stimulation with IL-5 induced modest upregulation of Bcl-2 mRNA and protein, with no significant modulation of the other Bcl-2 homologues examined. Caspases are the conserved executioners of the apoptosis. Eosinophils endogenously expressed 'initiator' caspase-8 and -9, and 'effector' caspase-3, -6, and -7. Spontaneous eosinophil apoptosis involved caspase-independent translocation of Bax to the mitochondria, resulting in perturbation of the mitochondrial membrane, cytochrome c release, and subsequent activation of caspase-3, -6, -7, -8, and -9. IL-5 inhibited constitutive eosinophil apoptosis at a site upstream of Bax translocation to the mitochondria, thereby preventing cytochrome c release and caspase activation. Eosinophils constitutively expressed the conformationally active form of Bax diffusely in the cytosol, but predominantly in the nucleus. Apoptosis induced by Fas receptor ligation involved detectable activation of caspase-3 and -8, and caspase-dependent Bax translocation to the mitochondria, supporting classification of eosinophils as a Type II cell in terms of apoptotic control. The data implicate Bax and mitochondria as pivotal regulators of eosinophil apoptosis in response to diverse stimuli.