To investigate the cytotoxic effects of 5-fluorocytosine on melanoma cells genetically modified with cytosine deaminase gene, the gene was transduced into the tumor cells with the retroviral method. The cytotoxicity effects of 5-fluorocytosine on the tumor cells were measured with the MTT assay and clonogenic assay. It was found that the prodrug 5-fluorocytosine had significant cytotoxic effects on melanoma cells transduced with cytosine deaminase in vitro. The IC50 value of 5-fluorocytosine on transgenic and nontransgenic melanoma cells was 572 microg/mL(-1) and 3870 microg/mL(-1), respectively. Our experiment demonstrated the potential value of the cytosine deaminase gene/5-fluorocytosine system in the treatment of melanoma.