The latex of 'Crown-of-Thorns' (Euphorbia milii var. hislopii, syn. E. splendens) has been shown to be a potent plant molluscicide that could be used against the snails which are intermediate hosts of Schistosoma trematodes. However, a comprehensive toxicological evaluation of the latex is necessary before its large-scale use in schistosomiasis control becomes possible. In fact, one cause for concern is the presence of tumor-promoting phorbol esters in several plants of the Euphorbiaceae family. Phorbol esters as well as a number of other known tumor promoters share the common property of inhibiting metabolic cooperation (i.e., exchange of low molecular weight molecules via gap junctions) between Chinese hamster V79 cells in monolayer cultures. The present study was undertaken to determine if latex of E. milii presents tumor promoter-like activity is this short-term in vitro assay. Samples of lyophilized E. milii latex were tested at a noncytotoxic concentration range (1, 10, 50 and 100 micrograms/ml) in three independent experiments. 12-O-Tetradecanoylphorbol-13-acetate (10 ng/ml) was used as positive control. In all three assays, E. milii latex consistently inhibited metabolic cooperation between V79 cells at concentrations > or = 10 micrograms/ml. These results that E. milii latex contains tumor-promoting substances. These findings suggest that the use of crude latex as a molluscicide may pose a carcinogenic hazard to people who are continuously exposed to the product.