Enzyme immunoassay (EIA) for the determination of compound K (C-K), a major metabolite of ginsenoside Rb1 (G-Rb1) from Panax ginseng root by intestinal bacterial flora, was explored. Bovine serum albumin (BSA) was coupled to the C-26 position on the unsaturated side chain of C-K. Beta-D-galactosidase was introduced at the C-26 position of the saturated side chain. Antiserum, obtained by immunization of rabbits with C-K-BSA conjugate, possessed high affinity and specificity toward C-K. The EIA for C-K by the double antibody method was established in the range of 0.1--100 ng/tube. Plasma C-K after the oral administration of C-K and G-Rb1 to rats was determined by the established EIA. C-K was rapidly absorbed from the gastrointestinal tract after the administration, then slowly decreased. On the other hand, C-K appeared late and was retained for a long period of time in the plasma after the administration of G-Rb1, which itself is hardly absorbed.