The two human colon carcinoma cell lines HT-29 and SW620, which stably express herpes simplex virus thymidine kinase (HSV-TK), are sensitized to the cytotoxic effects of the antiviral drug ganciclovir (GCV). Compared with HT-29 cells, SW620 cells were more sensitive to lower GCV concentrations (<1 μM), accumulated GCV triphosphate more rapidly, and incorporated higher levels of GCV into DNA. Following a 24-hr exposure to 10 μM GCV, bystander killing was as much as sixfold greater in SW620 cells than HT-29 cells. This bystander effect was dependent on the level of HSV-TK expression, the number of cells expressing HSV-TK, and the overall confluency of the cells. However, bystander killing did not correlate with gap junctional intercellular communication as determined by microinjection of Lucifer Yellow fluorescent dye. SW620 cells were coupled to <3% adjacent cells (compared with >50% for HT-29 cells), but were still able to transfer phosphorylated GCV to bystander cells as soon as 4 hr after drug was added. These results emphasize the importance of cell-specific metabolism in HSV-TK/GCV-mediated cytotoxicity and may suggest a novel mechanism for bystander killing.