Interleukin-8 (IL-8), a pro-inflammatory cytokine with potent neutrophil chemotactic activity, was studied for its effect on eosinophil migration responses, in vitro. Normal density eosinophils were isolated from healthy, non-atopic subjects <0.35 × 109 eosinophils/1) and individuals with various diseases associated with a blood eosinophilia (range 0.56 × 109-12.2 × 109 eosinophils/1). IL-8 prod need a dose-dependent migrational response for eosinophils from subjects with an eosinophilia, optimal at 10−8m (P < 0.01) and the major component of the migrationsl response was chemokinesis. On a molar basis, FL-8 (EC50∼ 10−10m) was 100-fold more potent than platelet activating factor (PAF), although a comparison of the migrational responses showed that at optimal concentrations IL-8 (10−8m) produced only 30% maximal responses stimulated by PAF (10−6m). In contrast, IL-8 tested over a wide concentration range had a negligible effect on eosinophils from normal subjects. A direct correlation between the total blood eosinophil counts for all subjects and the absolute magnitude of the migrational response to IL-8 (r= 0.727,P < 0.01 at 10−8m), PAF (r= 0.551, P < 0.03 at 10−6m) and N-formyl-methionyl-leucyl-phenylalanine (fMLP) (r= 0.689. P < 0.02 at 10−8m), suggested that heightened eosinophil migrational responses in inflammatory mediators may arise as a consequence of in vitro priming mechanism(s) associated with the development of an eosinophilia. In this regard, eosinophils derived from human cord blood mononuclear cells cultured in the presence of eosinophilopoietic cytokines IL-3 and IL-5, produced migrational responses to IL-8 and PA F. that were comparable with that of eosinophils from eosinophilic subjects. Furthermore, incubation of eosinophils from normal donors with IL-5 (optimal concentration 10−9m), significantly enhanced the subsequent migrational responses to both IL-8 (10−8m, P < 0.01)and PAF (10−8mP < 0.05). Therefore, the increased responsiveness of eosinophils from eosinophilic subjects may reflect in vivo priming by IL-5 and this phenomenon may contribute partly to the mechanism(s) by which eosinophils preferentially accumulate at sites of allergic inflammation.