A dammarane glycoside, designated compound Mx (C-Mx), was isolated from the hydrolysate of 20(S)-protopanaxadiol type ginsenosides containing G-Rb(3) from Panax notoginseng leaves with crude snailase. Its chemical structure was elucidated to be 20-O-beta-D-xylopyranosyl(1-->6)-beta-D-glucopyranosyl-20(S)-protopanaxadiol on the basis of spectral analysis. Its cytotoxicity against breast cancer cell line MCF-7 and effects on the sensitivity to doxocubicin of doxocubicin-resistant MCF-7 cells were also investigated. The new compound showed moderate cytotoxicity and partial reversal of doxocubicin resistance.