An activity of blood concerned in prothrombin activation is reported that was not previously recognized. Prothrombin-R was produced from a prothrombin derivative prepared with lysosomes after addition of various plasma fractions. The reaction was optimal at pH 7.9, and thrombin-C was not formed. Cohn fractions from either rabbit or human plasma generated prothrombin-R, although only fraction IV (alpha globulin plus) gave complete regeneration. Further purification of Cohn fractions was achieved with DEAE-cellulose chromatography. All fractions gave 100% yield of prothrombin-R. An immunoelectrophoretically pure alpha globulin had the highest specific activity. Partially purified gamma or beta globulin and albumin, also, were effective in that order. Immunologic data support the view that the alpha globulin content of these other fractions was responsible for generation of prothrombin-R. Most likely the active agent is an enzyme. Accordingly the plasma constituent which generates prothrombin-R is designated enzyme-R.