Formation of fibrinolysin from its inactive precursor in serum was observed under the following conditions: (a) by adding the specific antigen to serum from sensitized guinea pigs; (b) by mixing normal guinea pig serum with peptone, agar, hyaluronic acid, chondroitinsulfuric acid, glycogen, pneumococcal polysaccharides, and heparin.

Activation of profibrinolysin by these agents differs from chloroform or streptokinase activation in that it requires the presence of some serum constituent non-precipitable with the euglobulin fraction and destroyed by heating at 56°C.

The bearing of these observations on the mechanism of anaphylactic and anaphylactoid reactions is discussed. The findings reported support the concept that proteolysis is part of the process determining the release of histamine and other toxic products.

It is suggested that the presence of fibrinokinase may be responsible for the toxicity of serum induced in vitro by a number of agents.

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