One hundred plants in 43 families used in Malaysian traditional medicine were screened for tumour promoting activity using two methods, the technique of activation of latent Epstein-Barr Virus (EBV) in Raji cells and the irritation test on mouse ear. Extracts of only eight plants belonging to the Euphorbiaceae were found to possess EBV activation factor and to give a positive irritation test in mouse ear. These plants included Euphorbia tirucalli L., E. splendes, Jatropha podagrica, J. gossypyfolia L., Pedilanihus tithymaloides (L.) Poitt., Croton argyratus Bl., Exocoecaria agallocha L. and Codiaeum variegatum (L.) Bl. Seven of these plants are used internally in Malaysian traditional medicine. As such, they pose potential danger in the promotion of initiated cells of the mucosal tissue towards disease. Further studies are required to assess the epidemiological impact of these plants in the development of disease.