The possibility of dissolving a recent thrombus by the intravenous administration of a fibrinolytic enzyme was studied in a series of patients. Fibrinolysin (plasmin) was prepared in several ways from human and bovine plasma, and preparations were obtained from other laboratories also. Altogether 114 infusions of fibrinolysin were given to 37 volunteer patients, most of whom had incurable cancer. Thirteen episodes of acute thrombophlebitis, occurring in 10 patients of this series, were treated. In five of the eight cases in which the age of the thrombus was three days or less, complete resolution was obtained. When improvement did occur, the pattern of response was fairly uniform; pain was relieved within 24 hours, color improved and tenderness disappeared shortly thereafter, and edema disappeared within a few days. The results, which were sometimes striking, are illustrated in 14 case histories. Fibrinolysin was ineffective when given a week or more after the first manifestations of thrombosis, and it did not prevent the formation of fresh thrombi. The mechanism of its action upon blood clots in vitro as well as the clinical experiences here reported argue in favor of simultaneous administration of anticoagulants with fibrinolysin in the management of acute thromboses.