The tumor, induced by intramuscular inoculation of methylcholanthrene in the thigh of mice, was removed by amputation of the leg and the cell was isolated by mincing and trypsinizing the tumor fragment, and suspended in isotonic saline in counted number after viability was tested by trypan blue dye stayning. Mitomycin C (MMC) and Urokinase (UK) was administered either in vitro or in vivo and an effect was evaluated in the transplantability of the tumor cell which was inoculated in the subcutaneous tissue of the back of their own hosts. The results were summarized as follows: 1) Only when the tumor cell was inoculated with MMC, the concentration of which was minimally enough to lower the transplantability by itself, the addition of UK suppressed the take of the tumor, slightly more than that in MMC treatment. No evidence was obtained that the concentration of MMC below the above gained cytostatic effect by the addition of UK in vitro. 2) Definite suppression of take of the transplanted tumor cell and marked prolongation of interval between the transplantation and take were observed, when the combined intraperitoneal administration of MMC and UK was started in the day of the transplantation, compaired with that in mice MMC alone was samely given. The growth of the tumor was retarded in accordance with this suppression in the tumor take. It was also observed that the take of the tumor cell was diminished to some extent even in mice treated with UK alone. However, when the treatment was started 10 days after the transplantation, the growth of established tumors was not affected by the combined application of UK. It was concluded that adjuvant effect of UK in anticancer chemotherapy for prevention of tumor metastasis could be expected and an assumption was made that the mechanism lie in interruption of tumor cell lodgement through fibrinolysis caused by UK administration.