The herbal medicine "Saiboku-to" (ST) has been used for the treatment of bronchial asthma. ST was found to inhibit histamine release from and the degranulation of mouse peritoneal mast cells induced by compound 48/80. Its inhibitory effects were concentration-related and somewhat weaker than disodium cromoglycate, a known inhibitor of histamine release and mast cell degranulation. From these results, ST could appear to be useful in the treatment of Type I allergic reactions.