A new and accurate method to quantitate aflatoxins in medicinal herbs is developed. This method consists of extraction of the sample with MeOH-H2O (70:30) followed by clean-up of the extracts with immunoaffinity columns and, finally, high-performance liquid chromatographic determination with fluorescence detection. Aflatoxins B1 and G1 are determined as their bromine derivatives, produced in an online post-column derivatization system. The overall average recoveries for three different medicinal herbs spiked at levels of 1.3 and 2.6 ng/g of total aflatoxins range from 93% to 97%. The detection limit is 0.15 ng/g for both G2 and B2 and 0.20 ng/g for both G1 and B1, based on a signal-to-noise ratio of 3:1 and a precision (within-laboratory relative standard deviation) ranging from 0.8% to 1.4%. The use of immunoaffinity columns provides excellent clean-up of these particular extracts, which are generally difficult to analyze. The method is applied successfully to 96 samples of natural drugs.