1. Sterile cell-free filtrates of broth cultures of some strains of staphylococci contain a substance (staphylocoagulase) which does not clot purified fibrinogen, but does clot oxalated plasma.
2. When a plasma factor (coagulase globulin, CG) is added to staphylocoagulase a thrombin-like substance (coagulase-thrombin, CT) is progressively formed which is able to clot purified fibrinogen.
3. When the clotting times of plasma with increasing amounts of either staphylocoagulase or CT are plotted against concentrations of the clotting agents, hyperbolic curves are obtained which are similar to those obtained with classic prothrombin or thrombin.
4. CG appears to be distinct from AcG (Seegers), the "V" factor of Owren, and "anti-hemophilic globulin" of Taylor and co-workers. The presence of CG in platelets could not be demonstrated.