The Fifth Amendment states that "no person shall be compelled in any criminal case to be a witness against himself," meaning he or she may "plead the Fifth" under oath to protect the presumption of innocence and avoid criminal liability. If an individual accepts a presidential pardon, the privilege to plead the Fifth disappears, and the person could be compelled to testify.
An 1896 Supreme Court decision determined that “if the witness has already received a pardon, he cannot longer set up his privilege, since he stands with respect to such offense as if it had never been committed.”
In a 1915 decision, the court ruled a person can reject a pardon to protect his or her right against self-incrimination.