Eighteen herbs used in the treatment of infectious diseases in traditional Chinese medicine were evaluated for in vitro activity against ten microbial pathogens. Lyophilized teas were tested by the agar dilution technique at 100-1600 micrograms/ml. Eleven of the preparations were active against at least one microorganism and six of these were active against at least three of the test isolates. Huangqin (Scutellaria sp.) and Huanglian (Coptis sp.) were each active against five of the isolates. Huangqin inhibited Klebsiella pneumoniae and Proteus vulgaris at 200 micrograms/ml. Huangqin alone showed strong activity against Mycobacterium smegmatis (less than or equal to 100 micrograms/ml) and Candida albicans (200 micrograms/ml). The antimicrobial activity of various teas, prepared with equal weights of herbs, could be compared against a particular pathogen by considering both the percentage of water-soluble material in the herbs and the minimum inhibitory concentrations of the filtered, lyophilized decoctions.