The Kampo-prescription, Shi-un-kou, and its constituent crude drugs [Lithospermum erythrorhizon (1), Macrotomia euchroma (2) and Angelica acutiloba (3)] were assayed for their inhibitory effects on Epstein-Barr virus activation induced by the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). The crude drugs exhibited inhibitory activity singly and in combinations. In particular, the combination of 2 and 3 yielded enhanced inhibition and lower cytotoxicity. The anti-tumor promoter activity suggested by these results was further investigated in an in vivo study, which demonstrated that Shi-un-kou markedly inhibited TPA-induced skin tumor formation in mice.