Chios mastic gum (CMG) is a white, semitransparent, natural resin that is obtained as a trunk exudate from mastic trees. Triterpenic compounds and phytosterols like tirucallol are among its major components. CMG has been associated with cardiovascular protection, exerting its effect mainly through increasing the antioxidant defense system, and effectively lowering the levels of serum cholesterol in human subjects. However, data on its anti-inflammatory effect on endothelium are scarce. Attachment of leukocytes to the vascular endothelium and the subsequent migration of cells into the vessel wall are early events in atherogenesis, and this process requires the expression of endothelial adhesion molecules. In this study, we examined the effect of CMG neutral extract (25-200 microg/ml) and tirucallol (0.1-100 microM) on the following: 1) the expression of adhesion molecules (VCAM-1 and ICAM-1) by Cell ELISA and 2) the attachment of monocytes (U937 cells) in TNF-alpha stimulated Human Aortic Endothelial Cells (HAEC) by Adhesion assay. The impact of treatment with CMG neutral extract and tirucallol in NFkB phosphorylation was also examined by a cell-based ELISA kit. Both CMG extract and tirucallol inhibit significantly VCAM-1 and ICAM-1 expression in TNF-alpha-stimulated HAEC. They also inhibit significantly the binding of U937 cells to TNF-alpha-stimulated HAEC and attenuate the phosphorylation of NFkB p65. This study extends existing data regarding the cardioprotective effect of CMG, expands the spectrum of known phytosterols with potent antiatheromatic activity, provides new insight into the mechanisms underlying the beneficial effect of CMG on endothelial function, and may aid in design of new therapy for intervention in atherosclerosis.