Intratumoral gene transfer may be a significant tool in active immunotherapy. The ability to insert functional genes into a tumor in vitro and in vivo using recombinant vaccinia vectors was examined in the murine bladder tumor model. Vaccinia recombinants expressing the influenza hemagglutinin or nucleoprotein antigens infected/transfected murine (MB-49 and MBT-2) and human (T24) bladder tumor cell lines in vitro. Systemic vaccinia immunity was induced with as few as 10 plaque-forming units of recombinant vaccinia instilled intravesically, and the encoded protein was expressed in vivo in tumor and urothelium. However, preimmunity to vaccinia did not inhibit intravesical tumor transfection. Thus, recombinant vaccinia virus is effective in introducing foreign antigens locally into tumor in vivo, supporting its use in clinical immunotherapy.