The feasibility of gene therapy strategies in cancer treatment still has important pitfalls. Transfer of therapeutic proteins to the hypoxic/necrotic 'extracellular' microenvironment of solid tumors, based on the engineering of nonpathogenic clostridia is proposed as an alternative methodology. Using the rat rhabdomyosarcoma R1 in vivo tumor model, we demonstrated that Clostridium species colonized the tumors, whereas proliferation of these bacteria was absent in normal tissues. C. acetobutylicum was genetically engineered to express and secrete either mTNF-alpha or the E. coli cytosine deaminase. Quantitative in vitro data showed stability of the vectors, and significant levels of biologically active therapeutic proteins in lysates and supernatants of recombinant clostridia. Administration of either of these recombinant Clostridium strains to tumor-bearing rats resulted in the presence of active proteins in the tumor tissue. Based on these data and supported by its selective colonization pattern and safety, the Clostridium gene transfer system offers a potential application in anti-cancer therapies.