In an examination of 132 local strains of staphylococci† it was found that 80% of all strains originally isolated from internal human lesions are capable of liquefying human fibrin.1 Approximately 90% of all strains isolated from superficial human infections, however, and all strains isolated from veterinary lesions are non-lytic by the same in vitro technic.We have retested these and 9 additional local strains of Staphylococcus aureus or albus with fibrins isolated from lower animals. Data thus obtained are summarized in Table I.The table suggests that there are at least 5 different fibrinolytic factors formed by pathogenic staphylococci. Each lytic factor is apparently specific for the fibrin of one animal species. At least 3 of these species-specific fibrinolysins are apparently independent variables in staphylococci.Of particular interest are the 2 monovalent horse-lytic strains (Group C). These 2 strains were originally isolated by the Cutter Laboratory from “abscesses”in horses. With these 2 excepti...