Colicins are a family of antibacterial cytotoxins produced by Escherichia coli and released into the environment to reduce competition from other bacterial strains. Colicins kill the target cell by a variety of effects that include depolarisation of the cytoplasmic membrane, a non-specific DNase activity, a highly specific RNase activity or by inhibition of murein synthesis. This review summarises some important findings that implicate colicins as potential anti-tumor agents. Colicins appear to inhibit proliferation of tumor cell lines in a colicin-type--and cell line-dependent fashion and are more toxic to tumor cells than to normal cells within the body. This opens a potential for using bacterial colicins in combating cancer and raises a number of questions concerning the mechanism of action of colicins in targeting tumor cells, their specificity and applicability as anti-tumor therapeutics.