To establish a novel immunoassay for qualitative detection of ginsenoside Rb1 in rat serum.Anti-G-Rb1 monoclonal antibody (mAb) was through a hybridoma approach. Rat serum containing G-Rb1 was deproteinized with methanol to prepare the sample for testing, which was loaded onto polyethersulfone (PES) membrane and developed in the mixture of acetonitrile, water and acetic acid (25:75:1). After treatment with NaIO(4), the membrane was transferred to 1% BSA solution for immobilization of G-Rb1. The membrane was subsequently treated with anti-G-Rb1 mAb solution, followed by addition of peroxidase-labeled goat anti-mouse IgG and color development using 4-chloro-1-naphthol-0.03% H(2)O(2).On the PES membrane, a clear blue spot representing G-Rb1 occurred where the rat serum for testing and the standard G-Rb1 samples were blotted. The limit of this immunodetection was 0.25 microg.This immunoassay has greater specificity and reliability than thin-layer chromatography with a sensitivity similar to that of high-performance liquid chromatography, and does not require sophisticated equipment for convenient G-Rb1 detection in rat serum.