Inflammation is a frequent radiation-induced reaction following therapeutic irradiation. Treatment of human umbilical endothelial cells (HUVEC) with $gamma$-irradiation ($gamma$IR) induces the expression of adhesion proteins such as intercellular adhesion molecule-1 (VCAM-1 ), vascular cell adhesion molecule-1 (VCAM-1), and E-selectin. Since the upregulation of these proteins on endothelial cell surface has been known to be associated with inflammation, interioring with the expression of adhesion molecules is an important therapeutic target. In the present study, we demonstrate that high mannronic acid-containing alginate (HMA) inhibits $gamma$IR induced expression of ICAM-1, VCAM-1, and E-selectin on HUVEC in a dose dependent manner. HMA also inhibited $gamma$IR induced production of Nitric oxide (NO). These data suggest that HMA has therapeutic potential for the treatment of various inflammatory disorder associated with an increase of endothelial leukocyte adhesion molecules.