We have assessed the effect of combine cancer gene therapy with exogenous human tumor necrosis factor alpha (hTNFalpha) and Escherichia coli cytosine deaminase (CD) suicide gene on two human breast adenocarcinoma cell lines MDA-MB-361 and SK-BR-3. Transfection of a plasmid containing hTNFalpha under the control of a hybrid promoter resulted in expression of hTNFalpha gene in vitro. Transduction of retroviral plasmid containing bacterial cytosine deaminase led to the expression of cytosine deaminase in adenocarcinoma cell lines as well. The significant increase in apoptotic cells and decrease of cell proliferation in both tumor cell lines was observed using combination treatment with hTNFalpha expression plus CD/5-FC suicide system. Corresponding data were generated by MTT cell proliferation assay and by flow cytometric analysis. The presence of both genes after transduction of retroviral vector containing CD and transfection of hTNFalpha plasmid was confirmed by PCR and RT-PCR. The additive cell killing effect due to the presence of bacterial CD and hTNFalpha gene after activation of non-toxic prodrug was observed. Whether the bicistronic vector containing both therapeutic genes improve the therapy need to be assessed in the future.