AbstractThe distribution and cellular localization of Monophosphoryl Lipid A in the kidney, liver, lung, spleen, and stomach of rat were investigated by immunohistochemistry on paraffin embedded tissue sections. Rats were sacrificed 24h or 48h following intraperitoneal administration of MPL at a dose of 5mg/Kg. The presence of MPL in selected tissues was indicated by a positive reaction to MPL antibody. In the kidneys, MPL was found in collecting tubules and distal convoluted tubules in the medulla, whereas the glomerulus was essentially free of it. Regarding liver, MPL was found to be abundant in hepatocytes but only occasionally present in Kupffer cells. In lungs, both alveolar and bronchiolar macrophages were positive, indicating the lung can also serve as a possible site for the elimination of MPL. In spleen, endothelial cells and macrophages were positive for MPL. In stomach, MPL was detected in the gastric mucosa and vascular endothelial cells.In all the tissues studied the intensity of the peroxidase reaction was significantly weaker in 48h samples as compared to corresponding 24h samples indicating possible elimination of MPL from these tissues.