Intravascular hemolysis and ultrastructure of erythrocytes from liver sinusoids were studied during and after infusion of Escherichia coli endotoxin in Labrador retriever dogs. Endotoxin infusion caused hemoconcentration, and induced disseminated intravascular coagulation (DIC), characterized by a rapid drop of platelet numbers, a gradual consumption of coagulation factors and activation of fibrinolysis. Advanced DIC and circulatory shock gradually developed, and the animals died after 7-15 h. Plasma hemoglobin concentrations did not rise for several hours, but late in the experimental period a significant intravascular hemolysis was constantly found. Circulating adenosine diphosphate (ADP) did not appear. During shock, liver biopsies revealed accumulation of erythrocytes often disintegrated within distended sinusoidal lumina. In advanced shock the fragmented erythrocyte seemed to form occlusive masses within the vessels. A fibrin-like material frequently appeared adjacent to the red cells. However, it did not have the periodicity characteristic for fibrin, and the ultrastructure of the material was very similar to that inside the erythrocytes. None of these changes were induced by saline infusion in control animals. The lack of fibrin formation and the late development of intravascular hemolysis indicate that red cell breakdown was of little importance for the initiation and progress of DIC.