To explore a more efficient multi-gene antitumor treatment, we developed an adenoviral vector expressing both herpes simplex virus thymidine kinase (HSVtk) and human interleukin-2 (hIL-2) (AdCMVTKhIL2). Production of hIL-2 is expected to augment antitumor T cell and natural killer cell activity. Two separate cassettes expressing HSVtk and hIL-2, each under the control of the human cytomegalovirus (CMV) immediate early gene promoter, were inserted into the early 1 region of adenovirus type 5. This vector showed similar direct cytotoxicity towards infected rat medullary thyroid carcinoma (rMTC) cells as did the single gene vector, AdCMVtk. rMTC cells infected with the virus in vitro showed high sensitivity to ganciclovir. After infection with AdCMVTKhIL2 at 100 m.o.i. for 1 h, more than 20 000 U hIL-2 were produced during 24 h by 1 × 10(6) rMTC cells on day 2 and day 3. hIL-2 was also detected in the supernatants of primary cultures from tumors treated in vivo by the AdCMVTKhIL2 vector. Infected cells lost their tumorigenicity when transplanted subcutaneously into syngeneic rats, whereas all control animals developed tumors. More than 63% of tumors (19 out of 30 treated tumors) were destroyed when AdCMVTKhIL2 was injected intratumorally, compared with 38% when tumors were treated with AdCMVIL2, and 12% when treated with AdCMVtk, indicating an antitumor effect superior to that of each single vector given alone at the same dosage. These results indicate that the AdCMVTKhIL2 vector efficiently produces both HSVtk and hIL-2, and provides an enhanced antitumor activity.