Injection of ACTH or cortisone has no effect on the amount of fibrinolysin resulting from the activation of profibrinolysin by peptone.

Antifibrinolytic activity of serum is significantly increased by ACTH or cortisone but not by desoxycorticosterone or corticosterone. No increase in anti-fibrinolytic power of serum is produced, however, by the injection of ACTH or cortisone in splenectomized guinea pigs.

Splenin A has the same action on fibrinolysin inactivation as ACTH or cortisone. Splenin B has the opposite effect.

This action on the antifibrinolytic power of serum appears to be due to an acceleration of the rate of combination between the enzyme and its inhibitor and not to an actual increase in antifibrinolysin.

The possible significance of these observations is discussed.

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