Following an intravenous administration into rats of a shock-inducing dose of endotoxin (2 mg) the lipopolysaccharide (LPS) was demonstrated immunohistochemically (light and electron microscopy) and determined quantitatively (radio-labelled LPS) in the lung tissue and in isolated alveolar macrophages. At different times after LPS injection morphological investigations of the pulmonary tissue and alveolar macrophages were carried out. One hour after endotoxin treatment 3% of the alveolar macrophages were already LPS-positive. The maximum extent of the immunoperoxidase reaction for endotoxin (100% cells involved) was observed on day 3, the vast majority (98%) of the alveolar macrophages being LPS-positive still on day 14. 0.9% of the injected radio-labelled LPS preparation was found to be associated with lung tissue on day 3. By this time 0.173 microgram LPS/10(6) alveolar macrophages was detected. During the time of ultrastructural investigation endotoxin appeared in the lung only within cells. By their high capacity for storing endotoxin and their numerical superiority the mononuclear phagocytes are the leading LPS-positive cells in the lung, although granulocytes, endothelial cells, and alveolar epithelial cells were sometimes also involved. The accumulation of a high percentage of activated macrophages in the lung seen in the late stage of shock could represent at least one of the main factors leading to damage of pulmonary tissue. The correlation between appearance of LPS-positive macrophages and histological signs of lung tissue injury in the present investigation is striking.