Oldenlandia diffusa (OD) and Scutellaria barbata (SB) have been used in traditional Chinese medicine for treating liver, lung and rectal tumors while Astragalus membranaceus (AM) and Ligustrum lucidum (LL) are often used as an adjunct in cancer therapy. In this study, we determined the effects of aqueous extracts of these four herbs on aflatoxin B1 (AFB1)-induced mutagenesis using Salmonella typhimurium TA100 as the bacterial tester strain and rat liver 9000 x g supernatant as the activation system. The effects of these herbs on [3H]AFB1 binding to calf-thymus DNA were assessed. Organosoluble and water-soluble metabolites of AFB1 were extracted and analyzed by high-performance liquid chromatography (HPLC). Mutagenesis assays revealed that all of these herbs produced a concentration-dependent inhibition of histidine-independent revertant (His+) colonies induced by AFB1. At a concentration of 1.5 mg/plate, SB and OD in combination exhibited an additive effect. The trend of inhibition of these four herbs on AFB1-induced mutagenesis was: SB greater than LL greater than AM. LL, OD and SB significantly inhibited AFB1 binding to DNA, reduced AFB1-DNA adduct formation, and also significantly decreased the formation of organosoluble metabolites of AFB1. Our data suggest that these Chinese medicinal herbs possess cancer chemopreventive properties.