1. The mechanism of polymorphonuclear leukocyte (PMN)-dependent platelet adhesion to cultured endothelial cells induced by platelet-activating factor (PAF) was investigated to determine whether PMNs release or generate any factor(s) capable of inducing platelet adhesion, and the roles of prostacyclin and endothelium-derived relaxing factor (EDRF). 2. Cell-free supernatants, sonicates or rapid filtrates of PAF-stimulated PMN suspensions did not induce platelet adhesion to endothelial cells, but the PMN sonicates induced platelet adhesion when endothelial cells were pretreated with both aspirin and NG-nitro-L-arginine (L-NOARG). Its microphotograph showed that mainly platelet aggregates adhered to the endothelial cell surface. 3. Platelet adhesion induced by the PMN sonicates to aspirin- and L-NOARG-pretreated endothelial cells was dose-dependently prevented by OP-41483 (1-100 nM), and slightly by L-arginine (1 mM). The inhibition of platelet adhesion by OP-41483 and L-arginine was potentiated by their combination. 4. WEB 2170 (3 microM), a PAF antagonist, inhibited platelet adhesion induced by the PMN sonicates. However, PAF alone did not induce significant platelet adhesion to aspirin- and L-NOARG-treated endothelial cells. 5. Platelet adhesion induced by the PMN sonicates was not suppressed by AA-861 and indomethacin. However, both superoxide dismutase and catalase significantly inhibited platelet adhesion, and, in combination, their inhibitory effect was synergistically potentiated. Mannitol had no effect. It was also significantly inhibited by alpha 1-antitrypsin, whereas chymostatin and elastatinal had no effect. 6. PAF-induced platelet adhesion to endothelial cells in the presence of intact PMNs was not suppressed by indomethacin and AA-861, or by protease inhibitors. SOD alone, and in combination with catalase, caused a slight but significant inhibition, while catalase and mannitol by themselves had no effect.7. PMN-induced platelet adhesion was slightly inhibited by OP-41483 (100 nM). L-Arginine (1 mM)alone had no effect, but slightly potentiated the effect of OP-41483. This platelet adhesion was not accompanied by suppression of prostacyclin synthesis.8. The results with the PMN sonicates show that prostacyclin, EDRF and active oxygens are important modulators of intercellular interactions between platelets and endothelial cells. These results further suggest that the mechanism of intact PMN-dependent platelet adhesion is primarily through platelet endothelial cell interactions in which leukocyte-derived active oxygens play a role, but does not involve platelet-platelet interactions inhibitable by prostacyclin.