The fibrinolytic activity of 156 malignant and 36 benign solid tumors from autopsy and biopsy specimens was studied by the fibrin slide technique. The inhibitory activity against fibrinolysis was graded according to the lysis time of vascular tissues within the tumor. The results show that all malignant solid tumors, with the exception of prostate carcinoma, demonstrated varying degrees of inhibition of fibrinolysis. Persistently high inhibitory activity was found in squamous cell carcinoma of the esophagus, the respiratory tract, cervix uteri, and skin; carcinoma of uterus; colorectal carcinoma; small cell anaplastic carcinoma of lung; neuroblastoma, carcinoma of bile duct, while malignant tumors of the kidney show a lesser degree of inhibition. In contrast, with the exception of the hydatidiform mole, benign solid tumors show little or no inhibition. A similar absence of fibrinolytic activity is seen in metastatic disease. Further studies of the role of the fibrinolytic system in tumors seems warranted.