Abstract: A diterpenoid-lactone, white thin crystals, C₂₀H₂₄O₃, m/z: 312 (M⁺), mp 222～223℃, UVλ_max (EtOH)217 (logε4.36)nm, has been isolated from the ethyl acetate extract of the roots of Tripterygium wilfordii Hook. f., in a yield of 0.025‰. Its structure was elucidated by spectral analysis(UV, IR, MS, ¹HNMR and ¹³CNMR) and X-ray SCD. It is the known triptophenolide with revision of structure.Triptophenlolide was shown to have obvious inhibiting effects on lymphocyte and IgG (P<0.01) when mice and rats were given ig 1.5 mg/kg. The total complements in blood serum was increased. When BALB/C mice were given ig 1.5 mg/kg, the ear oedema induced-by dimethyl benzene was significantly inhibited (P<0.01); The ear oedema induced by croton oil in SD rats at a dose of ig 1.0 mg/kg was also significantly inhibited (P<0.05). The vitamine C content of the adrenal gland was reduced in mice at a dose of 1.5 mg/kg. The ig LD₅₀ of triptophenolide was greater than 30mg/kg.