After administration of bacterial lipopolysaccharide, there is an increase in the number of leucocytes which adhere to the endothelial cell surface of the hepatic vessels and pass through the endothelial layer by comparison with controls. There is also marked endothelial cell damage including intracytoplasmic oedema, increased numbers of autophagic vacuoles and dilatation of the intercellular junction in LPS-treated samples. The presence of immunocytochemical products of leukotriene (LTR) and tumour necrosis factor (TNF) was examined using in both LPS-treated and control samples. Immunoreactions of LTR which were seen in specific granules of neutrophils and monocytes attached to the endothelial cell surface may indicate the onset of endothelial cell damage. Positive immunoreactions of TNF on the endothelial cell surface, seen only in LPS-treated samples, indicate that TNF may enhance the passage of blood cells through the endothelia and also increase the endocytotic activity of the liver parenchymal cells, as revealed by the present marker experiment using horseradish peroxidase. Positive reactions of TNF in lysosomes of the endothelial cells suggest that they are able to produce TNF and transport it to the cell surface.