Bioassay-guided fractionation led to the isolation of three indolopyridoquinazoline alkaloids, 1-hydroxyrutaecarpine, rutaecarpine, and 1-methoxyrutaecarpine as the active principles of antiplatelet aggregation in vitro, from the chloroform-soluble part of the fruit of Zanthoxylum integrifoliolum (Rutaceae). 1-Hydroxyrutaecarpine exhibited antiplatelet activity induced by AA and showed an IC50 value of ca. 1-2 micrograms/ml.