Endotoxin or bacterial lipopolysaccharide (LPS) is a structural component of Gram-negative bacteria. It is believed to be the major pathogenic factor of Gram-negative sepsis, and may result in intravascular coagulation and in a shock syndrome that is characterized by thrombocytopenia, leucopenia, hypotension, fever, reduced delivery of oxygen, metabolic acidosis and ultimately death. We have previously shown that both endotoxemic pigs and patients with Gram-negative sepsis have elevated levels of platelet microvesicles in their blood, which indicates platelet activation. In this study, we have used flow cytometry and fluorescein-labeled chicken anti-human fibrinogen to evaluate the in vivo effect of endotoxin on platelet function in a porcine model. Endotoxin infusion in pigs caused impaired platelet function when platelets were stimulated with adenosine-diphosphate in vitro ( P < 0.001). We also found a similarly decreased platelet function in patients with Gram-negative sepsis. Since flow cytometry is a rapid method for determination of platelet function, this method may turn out to be a useful tool in clinical situations. Our results may contribute to our understanding of the bleeding problems that may occur in septic shock and in disseminated intravascular coagulation.