3H-labeled fibrinogen with a specific activity of 2.0 .times. 107 cpm[counts per minute]/mg was prepared by the method of reductive alkylation. The use of the 3H-fibrinogen as a substrate for detection of intracellular and extracellular fibrinolytic activity derived from cultures of benzo(a)pyrene-transformed Syrian hamster cell lines was examined. In cell free assays, 3H-fibrinogen enabled reliable quantitation of the fibrinolytic activity associated with neoplastic cells. The elevated extracellular fibrinolytic activity in the transformed cell lines as compared to normal hamster embryo cultures was demonstrated with this substrate. The ease with which large quantities of 3H-fibrinogen of high specific activity and prolonged half-life can be prepared makes the use of this substrate an attractive alternative to 125I-labeled fibrinogen.