Eighteen patients with renal-cell carcinoma have been investigated in an attempt to elucidate the ratio of active urokinase enzyme to urokinase antigen in the tumor and adjacent normal kidney. The tumor itself exhibited a significantly increased total fibrinolytic activity, an increase in the relative contribution of anti-urokinase IgG-inhibitable plasminogen activator activity and increased levels of urokinase antigen when compared to normal renal tissue. In tumor-adjacent tissue total fibrinolytic activity was also, but not significantly, increased, this increase being completely due to tissue-type plasminogen activator. Correlation of active urokinase-type plasminogen activator with urokinase antigen revealed that in tumor tissue the enzyme was present to more than 70% in its active form whereas in tumor-adjacent tissue and normal renal tissue only half of the enzyme appeared to be active. No correlation was obtained between urokinase antigen present in one of the 3 tissues investigated and plasma urokinase antigen.