Trichosanthin crystals grown at pH 5.4 are orthorhombic P212121 with cell dimension a = 38.23 angstrom, b = 76.18 angstrom and c = 79.12 angstrom. Each asymmetric unit contains one protein molecule of 234 amino acid residues (M. W. 26,000). The structure has been solved at 5 angstrom resolution by conventional MIR method with two heavy-atom derivatives. The preliminary result suggests that the protein molecule in acidic solution does not have dramatic conformational change, compared with that in alkali solution.