Simplified differential display of mRNA was applied to isolate and identify genes transcriptionally regulated in mouse liver by sho-saiko-to administration. A cDNA fragment up-regulated by sho-saiko-to was isolated and characterized. cDNA sequencing and subsequent database analysis revealed that the fragment showed significant sequence similarity with mouse testosterone 16-alpha-hydroxylase (cytochrome P-450[16alpha]) cDNA. The increased level of mRNA expression of cytochrome P-450(16alpha) in association with sho-saiko-to administration suggests the molecular mechanism of the chemopreventive effect of sho-saiko-to. This result indicates the usefulness of the mRNA differential display technique to investigate the molecular mechanism of Kampo medicine.