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Chinese and African Euphorbiaceae plant extracts were shown to have a markedly enhancing effect on Epstein-Barr virus (EBV)-induced transformation of human lymphocytes. When 5 X 10(5) cord blood lymphocytes were seeded into the semisolid agar medium immediately after EBV exposure, 3-10 times more colonies developed in the presence of the plant extracts at their optimal doses. When a smaller number of 5 X 10(4) cells were seeded, transformed colonies were also observed in the presence of the plant extracts but not in their absence. All of the colonies picked up from the agar medium were EBV-determined nuclear antigen (EBNA)-positive and showed the typical blastoid morphology. There were no colonies detected in the EBV-uninfected cultures with the extracts, indicating that the virus was required for the promotion by these plant extracts of this lymphocyte transformation. Euphorbiaceae plants are known to be employed as local herbal drugs in southern China and tropical Africa, and the possible role as a co-factor of the plant extracts in the development of nasopharyngeal carcinoma (NPC) and African Burkitt's lymphoma (BL) is discussed.