The gene for the bacterial enzyme carboxypeptidase G2 (CPG2) was expressed internally in mammalian cells. Mammalian-expressed CPG2 had kinetic properties indistinguishable from bacterially expressed CPG2. Human tumor cell lines A2780, SK-OV-3 (ovarian adenocarcinomas), LS174T, and WiDr (colon carcinomas) were engineered to express constitutively either CPG2 or bacterial β-galactosidase. These cell lines were subjected to a gene-directed enzyme prodrug therapy regime, using the prodrug 4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoyl-L-glutamic acid (CMDA). The lines which expressed CPG2 had enhanced sensitivity to CMDA. Comparing IC₅₀s, WiDr-CPG2 and SK-OV-3-CPG2 were 11–16-fold more sensitive, whereas A2780-CPG2 and LS174T-CPG2 were ∼95-fold more sensitive than the corresponding control lines. CPG2-expressing cells and control cells were mixed in differing proportions and then treated with prodrug. Total kill occurred when only ∼12% of cells expressed CPG2 with the WiDr and SK-OV-3 lines and when only 4–5% of cells expressed CPG2 with the LS174T and A2780 lines, indicating a substantial bystander effect. These results establish this CPG2 enzyme/CMDA prodrug system as an effective combination for the gene-directed enzyme prodrug therapy approach.