Interleukin 2 induced alterations in the morphology of bovine aortic endothelial cells in vitro. The changes observed in confluent cultures of bovine aortic endothelial cells included retraction and elongation of cells leading to enlarged gaps between cells quantified by image analysis. Purified IL-2 (1 U/ml medium) increased the gaps between endothelial cells 3-4-fold compared with control cultures. The effect was transient, since the cells reverted to their original morphology 6-12 hours after the removal of IL-2. Correlative scanning electron microscopy (SEM) studies using fresh bovine aorta showed a dose-dependent alteration of the endothelial surface by IL-2 characterized by rounding and elongation of endothelial cells and prominent perinuclear areas. Gaps between the endothelial cells were observed when aorta samples were incubated with 2 U of IL-2/ml of medium. This was confirmed by SEM, transmission electron microscopy and Evans blue dye staining. These results suggest that IL-2 caused morphological alterations in endothelial cells that enhanced the permeability of the vascular endothelium.