The fruit juice of Ecballium elaterium, used as a folk medicine in Turkey for the treatment of sinusitis, was investigated for its anti-inflammatory activity in mice. Fractions obtained from the juice were tested in mice for their effects on increased vascular permeability, as induced by HOAc ip. The active principle was isolated from the CHCl3 extract as well as the H2O-insoluble part of the fruit juice and identified as cucurbitacin B. This is the first report that cucurbitacin B has a significant anti-inflammatory activity.