Tricholin, a ribosome-inactivating protein isolated from the culture broth of Trichoderma viride, has been shownto exert fungicidal effects on Rhizoctonia solani through a multi-hit kinetic interaction. Tricholin causes a parallel cessation of growth, uptake of amino acids, and protein biosynthesis. The in vivo mode of action of tricholin on protein synthesis and cell growth appears to be attributed to the diminishing of the polysome formation in R. solani through damage to large ribosomal subunits. These results concur with previous data and prove that tricholin is an effective inhibitor of protein synthesis. The efficacy of tricholin as an antibiotic agent was estimated to have a duration of approximately 42 hours.In recent years, fungi have been used frequently to control plant diseases caused by other fungi. In most of these cases control of phytopathogens is effected through mycoparasitism1~3). The expression of mycoparasitism is either by penetration and formation of hyphae within the host hyphae, or by antibiotic effects exerted as a result of contact1}. Trichoderma species are antagonists of many soil-borne phytopathogenic fungi4~6) and have significantly decreased infection and disease through mycoparasitism. Successful control of phytopathogens including Rhizoctonia solani, and other fungi has been achieved7'8). The molecular mechanism of this antagonism is not yet well understood. Recently we have isolated an extracellular protein, tricholin, from a strain of T. viride, and the molecular action of tricholin was elucidated in vitro9). In this study we evaluated the antibiotic effect of tricholin on phytopathogenic R. solani, and the in vivo mode of action of tricholin vs. the host cell was examined. The results implied that tricholin is a potential fungicide for use in agriculture and medicine. Materials and Methods Cell Culture and TricholinLyophilized culture of R. solani (CCRC31252) obtained from The Culture Collection and ResearchCenter (Hsinchu, Taiwan) was soaked with sterile water for 30 minutes before transferred to potato dextrose agar plate (Acumedia Inc., Baltimore, U.S.A.), then incubated at 30°C for 24hours. Single colony of R. solani was picked and inoculated into 50 ml of potato dextrose broth (PDB) (Difco Lab., Detroit, U.S.A.)in 250-ml flask. The cells were maintained at 30°C, with constant agitation. Tricholin used in all experiments was isolated from the culture broth of T. viride (CCRC 32654) according to procedures of Lin et al.9). Fungicidal EffectQuantitative measurement of fungicidal activity was done by viable counts. R. solani cells were incubated in PDBmediumas described above with selected concentrations of tricholin. During the course of the test, samples of 0.5ml were taken at time intervals and were plated on drug-free potato dextrose agar. After incubation, viable fungal colony counts were determined. The number of target sites per cell