Lunarine, C35H31N3O4, an alkaloid from Lunaria biennis Moench and L. rediviva L., forms a hydriodide hydrate, crystals of which are orthorhombic, space-group P212121 with Z= 4 in a unit cell of dimensions a= 11·60, b= 16·53, c= 14·01 Å. The crystal structure and absolute stereochemistry have been determined by single-crystal X-ray analysis. Atomic co-ordinates and temperature factors were determined by Fourier and least-squares calculations and 2372 reflections were refined to R 0·115. The alkaloid contains a spermidine chain in which the terminal nitrogen atoms are present as amides of p-hydroxycinnamic acid residues that are linked to give a 3-oxohexahydrodibenzofuran system, the C4-unit in the spermidine chain being the nearer to the reduced ring of the latter system. The absolute stereochemistry is the reverse of that selected earlier (C. Tamura, G. A. Sim, J. A. D. Jeffreys, P. Bladon, and G. Ferguson, Chem. Comm., 1965, 485).