The immunosuppressive effects of dexmedetomidine, a highly selective and widely used alpha(2)-adrenoceptor agonist for sedation, analgesia, and stress management, are investigated in vitro. In the present study, the respiratory burst of human neutrophils separated from venous blood was evaluated with dexmedetomidine treatment after Escherichia coli stimulation. The effects of five concentrations of dexmedetomidine (1, 5, 10, 50, 100 mu g/mL) were evaluated by rhodamine in a flow cytometer. The nitric oxide (NO) production and nitric oxide synthase (iNOS) activity were also determined by using commercial kits. The results were compared to the positive control responses (respiratory burst without drug). We found that dexmedetomidine significantly suppressed respiratory burst, NO production, and iNOS activity after stimulation with E. coli, in a dose-dependent manner. The suppressive effects of dexmedetomidine on phagocytic activity of human neutrophils were associated with respiratory burst coupled with NO production.