Radioimmunoassay method for the determination of ginsenoside Rg₁, one of the major saponins of Panax ginseng root, was established. The side chain of ginsenoside Rg₁ was modified and coupled to BSA. ¹²⁵I-Labelled compound of ginsenoside Rg₁ was prepared from a ginsenoside Rg₁-tyramine conjugate. Immunization of rabbits with the hapten-BSA conjugate gave antiserum possessing high affinity and specificity toward ginsenoside Rg₁. Ginsenoside Rg₁ can be determined by the established radioimmunoassay method over a range of 250 pg-10 ng per tube.