We have shown that bacteria injected intravenously into live animals entered and replicated in solid tumors and metastases. The tumor-specific amplification process was visualized in real time using luciferase-catalyzed luminescence and green fluorescent protein fluorescence, which revealed the locations of the tumors and metastases. Escherichia coli and three attenuated pathogens (Vibrio cholerae, Salmonella typhimurium, and Listeria monocytogenes) all entered tumors and replicated. Similarly, the cytosolic vaccinia virus also showed tumor-specific replication, as visualized by real-time imaging. These findings indicate that neither auxotrophic mutations, nor vaccinia virus deficient for the thymidine kinase gene, nor anaerobic growth conditions were required for tumor specificity and intratumoral replication. We observed localization of tumors by light-emitting microorganisms in immunocompetent and in immunocompromised rodents with syngeneic and allogeneic tumors. Based on their 'tumor-finding' nature, bacteria and viruses may be designed to carry multiple genes for detection and treatment of cancer.