Ginsenosides, the major active component of American ginseng, were analyzed using eastern blotting with anti-ginsenoside Rb(1) and Rg(1) monoclonal antibodies (MAbs). Immunoassay-guided fractionation of the methanol extract of American ginseng and column chromatography led to the isolation of two new minor dammarane-type saponins, named quinquenosides Ja (1) and Jb (2). Their structures were elucidated to be 6-O-[α-L-rhamnopyranosyl(1→2)-β-D-glucopyranosyl]-20-O-[β-D-glucopyranosyl(1→4)-β-D-glucopyranosyl]-3β,6α,12β,20β-tetrahydroxydammar-24-ene (1) and 3-O-[β-D-glucopyranosyl(1→2)-β-D-glucopyranosyl]-20-O-{[α-L-arabinofuranosyl(1→6)-β-D-glucopyranosyl](1→6)-β-D-glucopyranosyl}-3β,12β,20β-trihydroxydammar-24-ene (2) on the basis of chemical and spectroscopic methods.