1. In a previous study, we showed that magnolol, a potent antioxidant derived from a Chinese herb, attenuates monocyte chemotactic protein-1 (MCP-1) expression and intimal hyperplasia in the balloon-injured aorta of cholesterol-fed rabbits. Expression of cell adhesion molecules by the arterial endothelium and the attachment of leukocytes to the endothelium may play a major role in atherosclerosis. In the present study, the effects of magnolol on the expression of endothelial-leukocyte adhesion molecules and the activation of nuclear factor kappa B (NF-kappa B) in tumour necrosis factor-alpha (TNF-alpha)-treated human aortic endothelial cells (HAECs) were investigated. 2. Pretreatment of HAECs with magnolol (5 microM) significantly suppressed the TNF-alpha-induced expression of vascular cell adhesion molecule-1 (VCAM-1) (64.8+/-1.9%), but had no effect on the expression of intercellular cell adhesion molecule-1 and endothelial cell selectin. 3. Magnolol (5 and 10 microM) significantly reduced the binding of the human monocytic cell line, U937, to TNF-alpha-stimulated HAECs (58.4 and 56.4% inhibition, respectively). Gel shift assays using the (32)P-labelled NF-kappa B consensus sequence as probe showed that magnolol pretreatment reduced the density of the shifted bands seen after TNF-alpha-induced activation. Immunoblot analysis and immunofluorescence staining of nuclear extracts demonstrated a 58% reduction in the amount of NF-kappa B p65 in the nuclei in magnolol-treated HAECs. Magnolol also attenuated intracellular H(2)O(2) generation in both control and TNF-alpha treated HAECs. 4. Furthermore, in vivo, magnolol attenuates the intimal thickening and TNF-alpha and VCAM-1 protein expression seen in the thoracic aortas of cholesterol-fed rabbits. 5. Taken together, these data demonstrate that magnolol inhibits TNF-alpha-induced nuclear translocation of NF-kappa B p65 and thereby suppresses expression of VCAM-1, resulting in reduced adhesion of leukocytes. These results suggest that magnolol has anti-inflammatory properties and may play important roles in the prevention of atherosclerosis and inflammatory responses in vivo.