Tetrandrine (TET), a bis-benzylisoquinoline alkaloid isolated from the dried root of Hang-Fang-Chi (Stephania tetrandra S. MOORE), is traditionally used in China for treating inflammation, hypertension and silicosis. In this study, our aim was to examine the anti-inflammatory mechanism of TET through measuring the inducible nitric oxide synthase (iNOS), cyclooxygenase-1, and -2 (COX-1 and COX-2) expression, cytokines (TNF-α, IL-4 and IL-8) formation, nitric oxide (NO) release and prostaglandin E₂ (PGE₂) generation in lipopolysaccharide (LPS)-induced human monocytic (THP-1) cells. Results showed that TET remarkably suppressed the LPS (1 μg/ml) induction of NO release and PGE₂ generation. It also significantly attenuated the LPS-induced transcription of proinflammatory cytokines (TNF-α, IL-4 and IL-8) in a dose-dependent manner. Furthermore, TET at 100 μM significantly blocked the LPS induction of iNOS and COX-2 expression, but not the COX-1. Taken together, these results suggest that TET exerts anti-inflammatory effects probably through the suppression of COX-2 and iNOS expression.