Abstract: The antibacterial actions of Huang-lien(the root of Coptis chinensis) and Hou-p'o(the bark of Magnolia hypoleuca) have been studied. The water soluble parts of Huang-lien and Hou-p'o showed more marked antibacterial activity against Bacillus anthracis, Clostridium welchii(isolated from a case of lamb dysentery), Micrococcus pyogenes var. aureus, Erysipelothrix rhusiopathiae and Pasteurella multocida(isolated from yak), but showed slight activity against Escherichia coli and no activity against Pseudomonas aeruginosa. The antibacterial activities of these two drugs were found to be stable to heat treatment at 121for 30 minutes. The active antibacterial substance of Huang-lien was apparently associated with P₂(corresponding to berberine hydrochloride), a substance precipitated by 1% HCl from the water extract of Huang-lien. The antibacterial action of Huang-lien against Bacillus anthracis was shown to be slightly affected in the presence of animal serum. A dark-brown substance precipitated by 1% HgCl₂ from the aqueous solution of Huang-lien, also showed antibacterial action as P₂.