The inactivation of ACTH by human plasma or euglobulin fraction activated by streptokinase in vitro was studied.The assay of ACTH was carried out by the method of adrenal ascorbic acid depletion of non-hypophysectomized rat treated withprednisolone 4 hrs. before used.The ACTH inactivating activity of human plasma was enhanced as the dosesof added streptokinase were increased, as in the case of caseinolysis. But, wheneuglobulin fraction was used in place of human plasma, this parallelism was notobserved, that is, the inactivating system of ACTH could be fully stimulated byless streptokinase.Plasmin inhibitors, such as N, N'-dicarbobenzyloxy-L-Lysyl-Ne-carbobenzyloxy-L-Lysine, inhibited the inactivation of ACTH by human plasma activated by streptokinase at the concentration of 25μg/ml in vitro, while e-aminocaproic acid did not inhibit the inactivation of ACTH by human plasma at such a high concentration as1000μg/ml.