The effect of platelet-activating factor (PAF) and platelet factor 4 (PF4) on the adhesion of isolated human eosinophils or eosinophilic cell lines (EoL-1, EoL-3) was examined. Both PAF and PF4 augmented eosinophil adhesion to plates coated with AB plasma or recombinant soluble intercellular adhesion molecule-1 (r-sICAM-1). These findings suggest that PAF and PF4 not only modulate chemotactic activity of eosinophils but also intensify the function of eosinophil adhesion. Since PAF and PF4 induce the expression of adhesion molecules (LFA-1 alpha, LFA-1 beta, CR3) on eosinophils, we could conclude that PAF or PF4 are closely related to eosinophil accumulation not only as chemotactic agents but also as augmentative agents for eosinophil adhesion through involvement in functional eosinophil adherence as well as surface expression of adhesion molecules on eosinophils.