The Epstein-Barr virus (EBV) is a ubiquitous viral agent known to be distributed among humans and suspected to be intimately associated with Burkitt's lymphoma and nasopharyngeal carcinoma. Recently the EBV activity has been shown to be induced by several chemicals and the EBV inducing agents in our environment have come to be recognized as a serious problem. Throughout our continuous screening of plant extracts against the EBV induction, active constituents of two plant species, Sapium sebiferum (Euphorbiaceae) and Edgeworthia papyrifera (Thymelaeaceae), which are very familiar to us as a roadside and garden tree, and a raw material of Japanese traditional paper, respectively, have been investigated. Three active principles, isolated from the leaves of S. sebiferum, have been identified as tigliane type esters, 12-O-hexadecanoyl-phorbol-13-acetate (1), Δ-5,6-7β-hydroperoxy derivative (2) and Δ-5,6-7-one derivative (3) of 1. Two active principles, 8 and 9, isolated from the stems of E. papyrifera, have been characterized as intramolecular orthoesters formed from parent polyhydroxy-acids, oxygenated derivatives of 1α-alkylresiniferonol-ω-oic acids. 1, 2, 8 and 9 highly induced the EBV early antigen in Raji cells. Of the four constituents, the activity of 9 was higher than that of TPA (12-O-tetradecanoyl-phorbol-13-acetate), known to be the most potent tumor promoter.