The clotting time of human and rat plasma was found to increase markedly after exposure to mushroom tyrosinase. The main, but not exclusive, effect of tyrosinase on the clotting system seems to be on fibrinogen, since the inactivating effect of tyrosinase can be greatly reduced by the subsequent addition of excess fibrinogen just before the clotting time of the plasma is measured. No evidence was found for any major effect of tyrosinase on any of the other components of plasma considered with the possible exception of prothrombin. Purified prothrombin is susceptible while prothrombin in plasma or serum is refractory to tyrosinase.

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