The coagulase-reacting factor (CRF) of human plasma has been purified, concentrated, and largely freed of prothrombin activity by Seitz filtration, absorbtion on hyflo-amphogel columns, controlled phosphate buffer elution, and fractional ammonium sulfate precipitation.

The purified CRF preparations localized between the beta and the gamma globulin position on paper electrophoresis, and the principal of two components observed on analytical ultracentrifugation gave sedimentation rates between 2.75 and 3.07.

Highly purified prothrombin preparations of Seegers effectively react with staphylocoagulase to lead to the coagulation of plasma.

The electrophoretic and ultracentrifugal properties of the human prothrombin preparations examined differ significantly from those of the purified CRF.

Derivatives of prothrombin, involving a loss of prothrombin activity, such as thrombin and "autoprothrombin," do not correspondingly lose in CRF function.

The hypothesis is proposed that CRF activity is resident in some component or components of the prothrombin molecule not involved in prothrombin function per se, and that therefore prothrombin and CRF activity may either parallel each other when the molecule is intact or diverge when the smaller units only are involved.

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