The effect of heparin and partially desulfated heparin derivatives on thrombin and PAF-induced adhesion of PMNs to the endothelium was studied either by a fluorescence image analysis or by 111In-labeled PMNs. The results observed with these two techniques consistently indicated that heparin and O-des-heparin inhibited PMN adhesion in a dose-dependent manner. Moreover, N-des-Hep and N-O-des-Hep, even if less effective, also inhibited the adhesion of PMNs when used at high concentrations. The effect of heparin and heparin derivatives was not directed to endothelial cells but rather to PMNs, as shown by the absence of inhibitory effects, when heparins were preincubated with endothelium.