Abstract: AIM: To set up a radio-ligand binding assay of LTC₄ for studying the effect of Gn-3(a stilbene polymer isolated from Gnetum parvifolium) on the LTC₄ receptor.METHODS: Guinea pig lung membrane was used as experimental materials, ³H-LTC₄ as radio ligand, FPL₅₅₇₁₂ as positive control drug and Gn-3 as test drug. Bioassay in vitro was used to determine the biological function of Gn-3. RESULTS: The binding of 3H-LTC₄ to its receptor was shown to be specific, saturable and reversible. The Ki and Bmax values were 2.7×10^-10 mol．L^-1 and 3.55×10^-13　mol．mg^-1 protein, respectively, at 30℃. Gn-3 was found to inhibit ³H-LTC₄ in competing for LTC₄ receptor. The IC₅₀ and Ki values were 6.39×10^-7 mol．L^-1 and 3.32×10^-7 mol．L^-1 respectively. Gn-3 was also found to be a LTC₄ receptor antagonist by bioassay in vitro. CONCLUSION: The binding of ³H-LTC₄ on guinea pig lung membrane showed a single binding site. Gn-3 is an effective LTC₄ receptor antagonist.