To enhance the specific cytotoxic effects caused by the transfer of the E. coli cytosine deamlnase (cd) and lISVl-tk to CEA (carcinoembryonic antigen)-produeing cells, the expression of the ed-tk fusion gene, driven by the CEA promoter, was investigated followed by treatment with 5-FC and GCV in combination with radiation. The expression vector pCEAcd-tk, based on pcDNA3, was introduced into CEA-produeing cells using liposomes. In CEA-producing cells, the CEA promoter could efficiently drive the expression of the fusion suicide gene. The expression activity of the E. colcoli ed gene driven by the CEA promoter was about tbree times higher than that driven by the CMVCMV promoter in transfeeted LoVo cells. A combination of 5-FC and GCV could cause higher cytotoxicity to the cells expressing CD and TK than the use of a single prodrug alone. The cytotoxic effect after combining the two prodrugs with radiation was the highest among all treatments in vitro. In vivo, the result of a subrenal capsule assay showed that tile inhibition rates for 5-FC (0.5mg/g) and GCV (0.1rag/g) to GLC-82 cells transfected witb pCEAcd-tk were 18.04% and 55.00%, respectively. A combination of the prodrugs at the same dose resulted in a 152.50% inhibition rate. In addition, the bystander effect exerted by the pCEAcd-tk/5-FC+GCV system in vilro was greater than that induced by cd/5-FC or tk/GCV alone.