Representatives of modern thrombolytic agents have been shown to possess certain in vitro clot accelerating activities, which a variety of experiments indicate to be due to a ‘Hageman-like’ effect. This activity is surface dependent and remarkably resistant to long keeping (in active plasmin) and to acid heating, unlike ‘surface factor’ (‘activation product’). The effect is quite powerful under certain test conditions, but it remains to be seen whether it is of any significance during the use of thrombolytic agents therapeutically. * Aided by a grant (H 1510—C7) from the N.I.H. Division of Research Grants and Fellowships, U. S. P. H. S. ** Appointment supported by the International Cooperation Administration, under the Visiting Research Scientists Program administered by the National Academy of Sciences of the United States of America.