The toxic major saponin from the starfish Echinaster sepositus, has been completely characterized. The structure is unique having as it does both a Δ7,3β,6β-dioxygenated-23-oxosteroidal moiety and a sugar moiety [β-D-glucopyranosyl-(1→2)-β-D-galactopyranosyl-(1→2)-β-D-glucuronopyranosyl] which bridges the C-3 and C-6 atoms of the steroid. The 3β-hydroxy-group of the steroid forms an O-acetal linkage with the glucuronate unit, while the HO–C(6‴) of the glucopyranosyl unit is attached to C-6 of the aglycone, forming a 6β-O-ethereal linkage.