Certain bacteria have emerged as biological gene vectors with natural tumor specificity, capable of specifically delivering genes or gene products to the tumor environment when intravenously (i.v.) administered to rodent models. Here, we describe procedures for studying this phenomenon in vitro and in vivo for both invasive and noninvasive bacteria suitable for exploitation as tumor-specific therapeutic delivery vehicles, due to their ability to replicate specifically within tumors and/or mediate bacterial-mediated transfer of plasmid DNA to mammalian cells (bactofection).