A neutral component has been isolated from Mercurialis leiocarpa and its structure determined by X-ray analysis as 3,3'-bis-(1,1'-dimethyl-2,2'-dioxo-4,4'-dimethoxy-5,5'-dihydroxy-5,5'-dimethoxycarbonyl-3-pyrroline).Mercurialis leiocarpa Sieb. et Zucc. [1], was used as an indigo dye in Japan between the 8th and 12th centuries. A neutral component has been isolated in crystalline form, and the structure determined by a single crystal diffraction analysis. The neutral component (1) (mp 266–268°, 「α」₀²⁰ = 0), was isolated in 0.03% yield as colourless needles from the methanolic extract of the fresh herb of Mercurialis leiocarpa.The molecular formula C₁₆H₂₀N₂O₁₀ was determined on the basis of the mass spectrum (m/z 400.1131) and elemental analysis. The IR spectrum showed the presence of a hydroxyl group (3475 and 3300 cm⁻¹), carbonyls (1760, 1745, 1700 and 1685 cm⁻¹) and an enol double bond (1645 cm⁻¹). The UV absorption spectrum gave no characteristic maxima. The ¹H (CDCl₃) and ¹³C (DMSO-d₆) NMR spectra were rather simple, which suggested 1 to be a dimer of a C₈H₁₀NO₅ unit. In addition to the carbonyl absorptions in the IR spectrum, the appearance of the skeletal carbons as singlets in the ¹³C NMR spectrum made the structural assignment difficult.The correct structure was established by a single crystal X-ray diffraction study as 3,3'-bis-(1,1'-dimethyl-2,2'dioxo-4,4'-dimethoxy-5,5'-dihydroxy-5,5'-dimethoxycarbonyl-3-pyrroline). The reddish colour of 1 in sodium methoxide-benzene and its decoloration on exposure to air were very similar to those of crysohermidine, isolated from Mercurialis perennis L. [2]. This seems to suggest the possibility of the transformation of 1 to crysohermidine.