Euphorbia milii (Euphorbiaceae) is a decorative plant used in gardens and living fences. In China, it has also been employed in herbal remedies for hepatitis and abdominal edema. Since E. milii latex--lyophilized or in natura--proved to be a potent plant molluscicide, its toxicity to non-target organisms has been comprehensively studied. Concerns on a possible tumor promoting activity have discouraged its use as a locally-available alternative molluscicide in schistosomiasis control programs. Two in vitro assays (inhibition of metabolic cooperation in V79 cells and Epstein-Barr virus induction in Raji cells) had suggested that E. milii latex contained tumor-promoting substances. This study was undertaken to verify whether the latex acts as a tumor promoter in vivo as well. A single dose of the initiating agent DMBA (400 nmol) was applied on the back skin of male and female DBA/2 mice. Testing for tumor promoting activity began 10 days after initiation. Tetradecanoyl phorbol acetate (TPA) (5 nmol, positive control), lyophilized latex (20, 60 and 200 microg per mouse) or acetone (vehicle control) were applied on mouse back skin twice a week for 20 weeks. In TPA-treated mice, papillomas were firstly noted during the 11th week, and by the 17th week all animals exhibited skin tumors. No tumors developed in mice treated with the solvent alone and in those exposed to latex. Findings from the present study therefore indicated that E. milii crude latex does not act as a tumor promoting agent on the mouse back skin assay.