We have assessed the effect of exogenous human tumor necrosis factor alpha (hTNFalpha) in three human cancer cell lines; MDA-MB-361 (breast adenocarcinoma), HCT 116 (colon carcinoma) and 8-MG-BA (glioma). In vitro transfection of a plasmid containing hTNFalpha under the control of a hybrid promoter resulted in expression of hTNFalpha gene in all three cell lines and secretion into the culture medium was seen with MDA-MB-361 cells. Flow cytometric analysis showed a significant increase in apoptotic and necrotic cells in MDA-MB-361 and to a lesser extent in HCT 116 cells. Increased apoptosis was confirmed by an increase in pro-caspase 3 activation. No effects of hTNFalpha expression were seen in 8-MG-BA cells. Intratumoral delivery of the hTNFalpha expression plasmid into MDA-MB-361 tumor xenografts grown in nude mice caused hemorrhagic tumor necrosis. This strategy may be a simple and promising gene therapy approach to the treatment of some human tumors.