Abstract: Homing bone marrow cells deposited at ATTC CRL-12424 can be transfected with a toxic gene which expresses a compound which alone, or in the company of a triggering agent, kills neighboring cancer cells in the bone marrow of a patient receiving the therapy. Toxic genes include cytotoxin such as thymidine kinase, immune stimulating compounds such as interleuken-2 and radiation repair inhibitors, such as Ku protein. The transfected cells can be administered directly to the site of the tumor or systemically, or regionally, intramedullary (into the marrow) through intravascular administration. The latter alternative permits the delivery of very high doses of the effective agent.