Abstract: Further studies on the active constituents in the bulbs of Allium macrostemon Bunge led to the isolation and structural determination of two new furostanol saponin macrostemonoside E and F. On the basis of chemical evidences and spectral analysis (UV, IR, ¹H-NMR, ¹³C-NMR and FAB-MS), the structure of macrostemonoside E(Ⅰ) was elucidated as (25R)-26-O-β-D-glucopyranosyl-5α-furost-20 (22)-ene-3β,26-diol-3-O-β-D-glucopyranosyl (1→2) β-D-glucopyranosyl (1→3)-β-D-glucopyranosyl (1→4)-β-D-galactopyranoside; macrostemonoside F(Ⅱ) was established to be (25R)-26-O-β-D-glucopyranosyl-5β-furost-20 (22)-ene-3β, 26-diol-3-O-β-D-glucopyranosyl(l→2)-β-D-galactoside. Preliminary pharmacological tests showed that both macrostemonoside E and F could strongly inhibit ADP-induced human platelet aggregation in vitro. The IC₅₀ of the former was 0.417 mM and that of the latter was 0.020 mM.