1. Sera separated from intravascular clots obtained postmortem showed no factor IX activity.
2. Evidence is presented suggesting that there are two forms of factor IX, intravascular and extravascular, and that there is an agent, named for convenience factor IX evolver (FNE) responsible for converting the former into the latter with the evolvement of factor IX in the serum.
4. The significance of these experiments and some related problems are discussed.