Three Astragalus saponins Ⅰ, Ⅱ, Ⅲ hav been isolated from roots of A. membranaceus Bunge (Leguminosae). Astragalus saponin I is the major constituent. The structure of astramembangenin C_(30)H_(50)O_5 obtained from saponin I by Smith's oxidative degradation with NaIO_4 followed by NaBH_4 reduction, was assigned as 1 by means of spectroscopic analysis (IR, ~1H NMR, ~(13)C NMR and MS) as well as chemical degradation. The configuration of C_(20) and C_(24) of 1 was established as S and R respectively by means o...