Calcium ions accelerated the activation of the proteolytic enzyme of plasma from its precursor in the globulin fraction. Calcium did not appear to potentiate fibrinolysis by active bovine or human plasma proteolytic enzyme, nor did it accelerate the activation of the precursor of this enzyme by chloroform or by streptokinase. Experiments with partially purified plasma proteolytic enzyme suggested that the acceleratory effect of calcium was mediated indirectly. Since the concentration of calcium which was effective was comparable to that present in plasma, it is possible that the phenomena reported are of biological significance.

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