We have previously demonstrated the therapeutic effect and efficacy of implantation of cells genetically modified to express cytochrome P450 2B1 in a nude mouse tumor model. The cells are encapsulated in polymerized cellulose sulphate and injected into preformed tumors. Upon administration of ifosfamide, the P450 enzyme converts the ifosfamide into antitumorigenic toxic metabolites at the site required, thereby significantly reducing tumor burden. Feline kidney epithelial cells were chosen for these studies, because they are easy to culture and can readily be transfected. However, these cells are not suitable for eventual use in human patients, since they are known to express endogenous retroviruses that are able to infect mammalian cells. They thus represent a safety risk. Here we describe the establishment of a human cell line that has been genetically modified to express the same cytochrome P450 construct and their characterization. The usefulness of mitomycin C treatment, both to protect the cells from the toxic metabolites that they produce and to incapacitate these cells from replicating, should they escape from the capsules, has also been investigated.