Trichosanthin (TCS) is the active component extracted from Tianhuafen, a traditional herbal medicine that has been used for abortion in China for centuries. It belongs to the type-I ribosomeinactivating protein (RIP) family and can inactivate the eukaryotic ribosome through its RNA N-glycosidase activity. Recentstudies have shown TCS to be multifunctional, its pharmacologicalproperties including immunomodulatory, anti-tumourand anti-HIV activities. The membrane-insertion property ofTCS is thought to be essential for its physiological effect, for itmust get across the membrane before it can enter the cytoplasmand exert its RIP function. In this paper, the membrane-insertionmechanism of TCS was studied. The monolayer experimentrevealed that TCS's membrane-insertion ability was dependenton low pH. Fluorescence spectroscopy using 1-anilinonaph-thalene-8-sulphonic acid as a probe showed that low pH mayinduce the conformational change of TCS that leads to thehydrophobic-site exposure, and the CD result showed that thisconformational change did not alter its secondary structure.Such conformational change leads to an intermediate state,called the molten globular state' by previous investigators. ThepH-dependent membrane insertion and conformational changewere related by the fact that the optimal membrane-surface pHneeded was the same for the two events. From these and otherresults, a membrane-insertion model was proposed.