['To investigate whether the protective effect of antithrombin III, which has been shown to exert beneficial effects during septic disorders, including reduction of endotoxin-associated leukocyte/endothelial cell interaction and capillary perfusion failure, is mainly based on its anticoagulant capacity or direct effects on the microvascular endothelium.', 'Animal study with three treatment groups.', 'Animal research facility.', 'Syrian golden hamsters, 6-8 wks old with a body weight of 60-80 g.', 'In skinfold preparations of hamsters, normotensive endotoxemia was induced by intravenous administration of 2 mg/kg endotoxin (lipopolysaccharide, 2 mg/kg). Antithrombin III (n = 7 animals; 250 units/kg) or tryptophan49-blocked antithrombin III (n = 6; 250 units/kg) was substituted intravenously 5 mins before lipopolysaccharide administration. Saline-treated animals (n = 11), receiving only lipopolysaccharide, served as controls. Tryptophan49-blocked antithrombin III binds to glycosaminoglycans at the endothelial surface to a significantly lower extent while retaining its progressive anticoagulant effects.', 'Compared with controls, antithrombin III significantly reduced lipopolysaccharide-induced arteriolar and venular leukocyte adherence (p < .01) and prevented depression of functional capillary density (p < .01), whereas tryptophan49-blocked antithrombin III failed to significantly improve both variables. As measured in vivo by a monoclonal fluorescein isothiocyanate-labeled anti-antithrombin III antibody and intravital microscopy, the lack of effect of tryptophan49-blocked antithrombin III was associated with significantly lower antithrombin III/endothelium binding coefficients after 1 hr, 3 hrs, and 24 hrs of endotoxemia (p < .01).'] Are antithrombin effects on endotoxin-induced microcirculatory disorders mediated mainly by its interaction with microvascular endothelium?