Summary: The coagulase-reacting factor (C.R.F.) is present in high dilutions of human and rabbit plasmas, the titer being influenced by the concentration of coagulase in the test. Plasmas of other animals are less effective when used in the unmodified state, but additional C.R.F. may be released by acid precipitation. All tissues examined reacted with coagulase, though titers lag behind that of plasma. Washed human platelets showed 0.1 per cent of the C.R.F. content of plasma. C.R.F. withstands 54 C for 30 minutes, but rapidly loses activity at higher temperatures. C.R.F. does not deteriorate rapidly on standing in plasma at ice-box temperature and even withstands 37 C for at least 1 week. C.R.F. is not adsorbed by Berkefeld, Chamberland, or Seitz filters, but is taken up by barium sulfate, magnesium hydroxide, and aluminum hydroxide. The possible relation of C.R.F. to the components of plasma concerned with the physiological clotting of blood is discussed.