Cytosine deaminase (CD) gene of E. coli converts the non-toxic compound 5-fluorocytosine (5-FC) into 5-fluorouracil. We have introduced a vector expressing the CD gene in a rat colon carcinoma cell line. Expression of the CD gene confers 5-FC sensitivity to these cells in vitro and in vivo. In a bifocal model consisting in a simultaneous engrafment of a CD+ tumor on one lobe of the liver and a wild-type parental tumor on the opposite lobe, treatment with 5-FC results in regression of both type of tumors, indicating the existence of a distant bystander effect.