A monolayer technique was used to investigate the interaction between the ribosome inactivating protein trichosanthin (TCS) and phospholipid membrane.The adsorption experiments show that the negatively charged 1,2-dipalmitoyl-sn-glycerol-3-phosphoglycerol (DPPG) causes obvious enrichment of TCS beneath the monolayer, indicating electrostatic attraction between TCS and the negatively charged phospholipid.When TCS was incorporated into the DPPG monolayer at low pH, it could not be completely squeezed out until the monolayer collapsed.The results suggest that the electrostatic attraction and the hydrophobic force are involved in the interaction between TCS and phospholipids at different stages.These findings may be correlated with the membrane translocation mechanism of TCS.