The ultrastructural appearance of normal skin following epicutaneous application of leukotriene B4 (LTB4) was studied using transmission electronmicroscopy. The acute phase (first 24 h) after LTB4 challenge is characterized by extravasation of polymorphonuclear leukocytes (PMN) and endothelial changes such as focal necrosis, formation of fenestrations, gaps, and a multilayered basal lamina. In the 'late phase' (24-48 h), the endothelium still possesses many fenestrations and gaps. The density of endothelial cell nuclei is increased, and the relatively large nucleoli and high density of mitochondria are indicative of a hyperactive endothelium. The PMN in this phase show pycnotic nuclei and a vacuolated cytoplasm. Macrophages are observed phagocytosing these degenerating PMN. Monocytes and lymphocytes extravasate without any signs of endothelial or epidermal damage in their vicinity. The mast cells show no degranulation throughout the observation period.