It had been previously demonstrated that fibrin clot might have an important role in the growth of malignant tumors and in metastasis formation. For this reason an attempt had been made to find out whether inhibition of fibrin formation by heparin or fibrinolysis induced by defibrase could limit tumor growth. Male mice of inbred strain were challenged with suspension of tumor cells. For anticoagulation and/or fibrinolytic therapy two different schedules were used. The treatment was started either simultaneously with the tumor transplantation, or one week prior to the injection of tumor cells. The size of tumors was bigger in the group of treated animals, when compared with the controls. However, the difference was not statistically significant. On the other hand, frequency of implanted tumors was significantly higher in animals with heparin and defibrase treatment or pretreatment.