The alkaloid cryptolepine is thought to mediate the anti-inflammatory effects of the climbing shrub, Cryptolepis sanguinoleta. The underlying mechanism of action, however, is largely unknown. In the present study, we show that the synthetic cryptolepine-hydrochloride (2.5-10 μM) dose-dependently inhibits lipopolysaccharide (LPS)-induced nitric oxide production in the murine macrophage cell line RAW 264.7. We furthermore demonstrate a strong inhibition of nuclear factor (NF)-κB, a transcription factor primarily involved in inflammatory and immune responses, by cryptolepine (2.5-10 μM) using a luciferase reporter gene assay in human HEK 293 cells. Examining the individual steps of NF-κB activation in the presence of cryptolepine we could exclude an inhibitory effect on degradation of IκB or nuclear translocation of NF-κB by the alkaloid. However, EMSA of nuclear extracts from LPS-activated RAW cells revealed reduced DNA binding activity of NF-κB by cryptolepine in vivo and in vitro. This indicates that cryptolepine may exhibit its anti-inflammatory action by blocking DNA binding of activated NF-κB and thus transcription of NF-κB-regulated proinflammatory proteins.