The invention intends to find out an in-vitro bioassay system capable of ensuring qualities of yokukansan to a higher degree, and provides a bioassay method for yokukansan, comprising competitively reacting labeled ligand and a test sample containing yokukansan with cells or cell membranes expressing serotonin 1A receptors, and measuring binding activity of yokukansan from the amount of the labeled ligand bound, and a bioassay method for yokukansan, comprising reacting labeled GTP and a test sample containing yokukansan with cells or cell membranes expressing serotonin 1A receptors, and measuring receptor-agonist activity of yokukansan from the amount of the labeled GTP bound.
