After priming with cytokines, such as granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-3, or IL-5, eosinophils are stimulated potently by opsonized particles like serum-treated zymosan (STZ), resulting in activation of the respiratory burst and production of lipid mediators, such as platelet-activating factor (PAF) and leukotriene C4 (LTC4). In the present study, the role of the opsonin receptors Fc gamma RII and CR3 during both STZ-induced activation of the respiratory burst and PAF release by human eosinophils was investigated. Inhibition studies with blocking mAbs (alpha hFc gamma RII: AT10, IV.3; alpha CR3: B2.12, 44a) showed that both Fc gamma RII and CR3 are important for STZ-induced PAF release by cytokine-primed eosinophils. In contrast, CR3 is involved in activation of the respiratory burst, whereas Fc gamma RII seems not to be important, because blocking anti-Fc gamma RII mAbs had no effect. Subsequently, experiments were performed with zymosan particles coated with IgG, iC3b, or a combination of both. IgG-coated particles poorly activated both responses in GM-CSF primed and unprimed cells. iC3b-Zymosan activated the respiratory burst as well as zymosan expressing both opsonins (IgG/iC3b-zymosan). In contrast, iC3b-zymosan induced significantly less PAF release by GM-CSF-primed eosinophils than did IgG/iC3b-zymosan, suggesting synergism between Fc gamma RII and CR3. This synergistic effect was not observed when IgG-zymosan and iC3b-zymosan were added simultaneously. Therefore, these data indicate that on human eosinophils, Fc gamma RII and CR3 act synergistically to activate PAF release, provided that their ligands are in close proximity.