The effects of the compositional drugs of kanpou medicine on the growth and aggregation of calcium oxalate crystals were investigated in vitro. In the seeded crystal system, the inhibitory activities were calculated from the change of the number (Ia), total volume (Ig) and fractional volume (I) of crystals determined by a Coulter Multisizer. Among the tested drugs (Takusha, Akyou, Chorei, Bukuryou, Kasseki, Kinsensou, Kagosou), only Takusha and Kagosou had strong inhibitory activities on the growth and aggregation at the concentration of 5 micrograms/ml and 10 micrograms/ml (Ia greater than 80%, Ig, I greater than 90%). No significant correlation was observed between inhibitory activities and alcian blue precipitable polyanions (ABPP) of each drug. Thus some naturally existing substances composing the kanpou medicine may help prevent the stone formation in clinical use.