BACKGROUND: The potential use of variola virus as a biological weapon has renewed efforts in the development of antiviral agents against orthopoxviruses. ST-246 [4-trifluoromethyl-N-(3,3a,4,4a,5,5a,6,6a-octahydro-1,3-di oxo-4,6-ethenocycloprop [f]isoindol-2(1 H)-yl)-benzamide] is an anti-orthopoxvirus compound active against several orthopoxviruses including vaccinia virus (VV), cowpox virus (CPV), camelpox virus (CMLV), ectromelia virus (ECTV) and variola virus in cell culture. The compound has been shown to inhibit the release of extracellular virus by targeting the F13L W protein and to protect mice from W, CPV and ECTV orthopoxvirus-induced disease. METHODS: The antiviral activity of ST-246 was assessed against extracellular and intracellular W, CPV and CMLV production in human embryonic lung (HEL) fibroblasts and primary human keratinocyte (PHK) cell monolayers, as well as in three-dimensional raft cultures. RESULTS: ST-246 inhibited preferentially the production of extracellular virus compared with intracellular virus production in HEL and PHK cells (for W) and in PHK cells (for CMLV). In organotypic epithelial raft cultures, ST-246 at 20 microg/ml inhibited extracellular W and CMLV production by 6 logs, whereas intracellular virus yield was reduced by 2 logs. In the case of CPV, both extracellular and intracellular virus production were completely inhibited by ST-246 at 20 microg/ml. Histological sections of the infected rafts, treated with increasing amounts of drug, confirmed the antiviral activity of ST-246: the epithelium was protected and there was no evidence of viral infection. Electron microscopic examination confirmed the absence of intracellular enveloped virus forms in W-, CPV- and CMLV-infected cells treated with 10 microg/ml of ST-246. CONCLUSIONS: These data indicate that ST-246 is a potent anti-orthopoxvirus compound; the mode of inhibition is dependent on the virus and cell type.