Human plasminogen, plasma, or serum increased the pathogenicity of six streptokinase-positive streptococcal strains for mice. Combinations of commercial streptokinase and plasminogen or streptokinase and plasma did not usually increase mouse mortality to a greater degree than did plasminogen or plasma alone, suggesting that the maximal effective amount of streptokinase was produced by the organisms.

The pathogenicity of Salmonella paratyphi (fibrinolysin variable) and a Group D streptococcus (streptokinase-negative) was not increased by plasminogen, plasma, or streptokinase, whereas a combination of streptokinase and plasminogen, or streptokinase and plasma did significantly increase mouse mortality resulting from these organisms.

Combinations of certain concentrations of streptokinase and plasminogen increased the pathogenicity of a fibrinolysin-negative staphylococcal strain for mice to a greater extent than did either substance alone.

The observed results provide evidence that streptokinase, by an interaction with plasminogen, contributes to the pathogenicity of streptokinase-positive streptococci.

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