Eosinophils are characteristic participants in allergic inflammation. The intracellular signalling mechanisms involved in the migration of eosinophils to sites of allergic inflammation are poorly understood. Chemotactic responses of eosinophils to platelet-activating factor (PAF), but not eotaxin, have been demonstrated to be dependent upon the activation of phosphoinositide 3-kinase (PI3K) but the specific isoform of PI3K involved has not been identified.To determine the roles of the leukocyte-specific PI3K gamma and PI3K delta isoforms of PI3K in PAF-induced chemotaxis of human eosinophils.Chemotactic responses of the EoL-1 eosinophilic cell line and human peripheral blood eosinophils were measured. The effects of a PI3K gamma-selective inhibitor (5-[2,2-difluorobenzo(1,3)dioxol-5-ylmethylene]-thiazolidine-2,4-dione; AS604850) and gene knock-down of PI3K gamma and PI3K delta on chemotactic responses were determined.AS604850 caused a concentration-dependent suppression of chemotactic responses of EoL-1 cells and blood eosinophils to PAF but not eotaxin. Specific siRNAs reduced the expression of PI3K gamma and PI3K delta in EoL-1 cells. Knock-down of PI3K gamma by siRNA resulted in a 75% inhibition of the chemotactic response to PAF but had no effect on the response to eotaxin. Knock-down of endogenous PI3K delta by siRNA resulted in a 38% inhibition of the chemotactic response to PAF but had no effect on the response to eotaxin.PI3K gamma plays a major role in the induction of chemotaxis in PAF-stimulated eosinophils, while PI3K delta plays a lesser role. Interventions which reduce the activity of PI3K gamma may have therapeutic potential in allergic diseases.