Heat durability of Epstein-Barr virus-activating substances of plant origin: 12-O-tetradecanoylphorbol-13-acetate, 12-O-hexadecanoyl-16-hydrophorbol-13-acetate, croton oil, tung oil and Croton megalocarpus extract.

12-O-Tetradecanoyl-phorbol-13-acetate (TPA), 12-O-hexadecanoyl-16-hydrophorbol-13-acetate (HHPA), croton oil, tung oil and Croton megalocarpus ether extract, all of which are known to possess Epstein-Barr virus-activating potency, retained their capacity to induce Epstein-Barr virus early antigen complex in the viral genome-carrying human lymphoblastoid cells (Raji) even after heat treatment at 120 degrees C for 2 h or at 100 degrees C for 12 h. Such unusual heat resistance of the agents tested may contribute to the persistence and accumulation of the active principle(s) in soil under the plants which contain such substances, as we have previously reported.