Soil-extracts collected from the ground from under several Euphorbiaceae plants have been known to possess Epstein-Barr virus (EBV)-activating substances which are thought to be one of the environmental co-factors causing nasopharyngeal carcinoma (NPC) in southern part of China and Burkitt's lymphoma (BL) in tropical Africa. Then, a model experiment aimed at chemical characterization of such active substances was carried out using a soil-extract around Sapium sebiferum, a Japanese representative Euphorbiaceae plant. Chromatographic separation guided by the EBV early antigen (EA) inducing activity gave a highly active fraction. Application of this fraction to desorption chemical ionization mass spectrometry identified a major active substance to be 12-O-hexadecanoylphorbol-13-acetate (HPA), which originally occurs in this plant. The method in this model experiment is suggested to be applicable to other samples from the endemic areas of NPC and BL.