The expression of Selectin-P was measured in terms of formation of "rosettes" by human gel-filtrated thrombin (30-50 mU)-stimulated platelets on the surface of isolated homologous neutrophilic leucocytes (PMNs) according to Jungi (1986). The monoclonal anti-Selectin-P antibody completely prevented the formation of "rosettes", proving the specificity of Selectin-P-mediated adhesion. The Selectin-P-mediated adhesion of platelets to PMNs was inhibited by both iloprost (ILO) (IC50 = 5.0 nM) and sodium nitroprusside (NaNP) (IC50 = 0.93 microM); thus ILO is ca. 180 times more potent an inhibitor of "rosette" formation than NaNP. The NOS-inhibitors L-NO2Arg (10-30 microM) and L-MetArg (3-30 microM) each suppressed the adhesion, while at lower and higher concentrations these NOS-inhibitors did not influence rosette formation. L-Arginine (up to 1 mM) was not able to influence significantly the Selectin-P-mediated adhesion of platelets to PMNs. The COX inhibitor aspirin (10-30 microM) promoted the adhesion. We conclude that the Selectin-P-mediated adhesion of platelets to PMNs is inhibited by both ILO and NaNP, whereas endogenous prostanoids and nitric oxide seem to exert an antagonist effect on the adhesion of platelets to PMNs.