The clinical success of suicide gene therapy using herpes simplex virus type 1 thymidine kinase (TK) is largely dependent on the capacity of this enzyme to effectively induce the death of bystander cells. We have shown that fusion of TK to an 11-amino acid peptide from the basic domain of the human immunodeficiency virus type 1 Tat protein (Tat11) imparts cell membrane-translocating ability to the enzyme and significantly increases its cytotoxic activity. Here we report on the efficacy of this strategy in two different mouse models of adoptive tumorigenesis, based on the implantation of human Kaposi sarcoma (KS-IMM) cells in nude mice or of B16F10 melanoma cells in syngeneic C57BL/6J mice. Experiments were performed by the subcutaneous injection of mixtures of unmodified tumor cells containing different fractions of TK or Tat11-TK producing cells followed by animal treatment with ganciclovir (GCV). In both systems we consistently found that mice bearing tumors containing Tat11-TK cells displayed significantly retarded tumor growth and prolonged survival as compared with mice inoculated with cells expressing unmodified TK. Collectively, these results demonstrate that fusion of Tat11 to TK imparts remarkable intercellular trafficking capability to the enzyme. This modification of TK might constitute an important step in the optimization of TK suicide gene strategy for gene therapy of cellular proliferation.