Vaccinia melanoma oncolysate (VMO) prepared by infecting B16F10 melanoma cells with recombinant vaccinia virus encoding murine GM-CSF gene was tested for its therapeutic effect on the preestablished melanoma. C57BL/6 mice were inoculated s.c. with 1 × l0⁵ B16F10 melanoma cells and received s.c. administration with VMO prepared with GM-CSF gene-encoded vaccinia virus(GM-CSFVMO), VMO prepared with thymidine kinase gene-deficient vaccinia virus(TKVMO), B16F10 melanoma oncolysate(BMO), or PBS 3 days after tumor inoculation. The same treatment was bolstered one week later. The results demonstrated that GM-CSFVMO treatment significantly inhibited the growth of subcutaneous tumor and prolonged the survival period of tumor-bearing mice. Further study elucidated that cytotoxicity of PBL and splenocytes towards B16F10 increased obviously after treatment with GM-CSFVMO, but NK activity remained unchanged. These results suggest that the tumor oncolysate vaccine prepared with GM-CSF gene-encoded vaceinia virus might exert potent therapeutic effect on the preestablished tumor through the efficient induction of specific antitumor immune response of the host.