The activation of profibrinolysin in sensitized guinea pig serum when mixed in vitro with the homologous antigen was confirmed with a more accurate and more reliable method than the one previously used. A study was made of some of the conditions required for obtaining maximum activation.

Profibrinolysin activation was also induced in normal guinea pig serum by addition of certain "anaphylactoid" agents such as peptone, tween 20, morphine, octylamine, octadecylamine, and 48/80.

The specific antigen and the anaphylactoid agents produce activation only when added to whole, fresh, unheated serum. Profibrinolysin activation by these agents, as opposed to activation by streptokinase, seems to require the intervention of a kinase system (serofibrinokinase) inactivated by fractionation of serum and by heating to 56°C.

Whenever serum was submitted to treatments which caused fractionation, fixation or inhibition of complement, serofibrinokinase was also inactivated. Under the conditions investigated the behavior of this kinase was indistinguishable from that of complement.

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