To search for possible anti-tumor-promoters (cancer chemopreventive agents), we carried out primary screening of 23 triterpenoid hydrocarbons (1-23) isolated from ferns using an in vitro synergistic assay system. Of these triterpenoids, hop-17(21)-ene (2), neohop-13(18)-ene (3), neohop-12-ene (4), taraxerane (17), multiflor-9(11)-ene (18), multiflor-8-ene (19), glutin-5(10)-ene (21) and taraxastane (23) exhibited remarkable inhibitory effects on Epstein-Barr virus (EBV) activation induced by the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). Further, compounds 2 and 3 exhibited remarkable anti-tumorpromoting effects on mouse skin tumor promotion in an in vivo two-stage carcinogenesis test using 7,12-dimethybenz[a]anthracene (DMBA) as an initiator and TPA as a promoter.