Short-term in vitro assays for tumor promoters and anti-tumor promoters (Epstein-Barr virus activation test) were carried out for 45 quassinoids. As a result, some quassinoids showed potent activity, more than 50% inhibition at a molar ratio of 1:1 (TPA/quassinoids). These results led to the following structure-activity relationships: (1) a methyleneoxy bridge and side chain enhance the activity and (2) a sugar moiety reduces the activity.