Trichosanthin was coupled with bromodextran and the reaction mixture chromatographed on Sephadex G-75 fine to yield two peaks. The first peak was judged to be bromodextran-trichosanthin by SDS-polyacrylamide gel electrophoresis. The yield was optimal when 4% trichosanthin and 8% bromodextran T20 were reacted for 210 hours. Bromodextran-trichosanthin exhibited an ultraviolet absorption spectrum similar to that of free trichosanthin and the complex reacted positively with the trichosanthin antiserum. It had lower abortifacient and protein synthesis inhibiting activities but it also possessed lower allergenicity, suggesting that it may be useful as a substitute of trichosanthin, especially when the problem of hypersensitivity caused by prolonged administration of trichosanthin is serious.