The invention provides a method of ameliorating a blood-brain barrier disorder, as well as a method of treating a disease accompanied by a blood-brain barrier disorder and a method of treating a cerebral ischemic disease, by administering a prothymosin &alpha；, or a protein or polypeptide possessing the same function as the prothymosin &alpha；. The invention also provides polypeptides useful in the context of the aforesaid methods.
