In the case of New Jersey v. Portash, the U.S. Supreme Court ruled in 1978 that a defendant's testimony from a legislative hearing could not be used against them in a criminal trial if they had been granted immunity at that hearing. The court held this to be true even if the testimony was being used only for impeachment purposes (to challenge the credibility of the defendant). Gerald Portash, who was under investigation for bribery and perjury charges, testified before a grand jury after receiving an assurance of immunity from prosecution based on his statements. Later during his trial when he denied making certain admissions about bribes, prosecutors sought to use his previous immunized testimonies as evidence contradicting these denials - essentially using it to impeach him as witness in his own defense. However, Justice William Brennan writing for majority argued that Fifth Amendment protection against self-incrimination extends beyond just preventing compelled incriminating testimonies from being used directly as evidence; it also prevents such testimonies given under grant of immunity from being employed to impeach credibility.

In the dissenting opinion for New Jersey v. Portash, Justice William Rehnquist argued that the majority's decision to exclude a defendant's grand jury testimony from his trial was incorrect. He contended that the Fifth Amendment privilege against self-incrimination does not apply when a defendant testifies under immunity at a grand jury proceeding and then contradicts this testimony at trial. According to him, using such contradictory statements as impeachment evidence would not be compelling the accused to incriminate himself but rather preventing him from committing perjury or presenting false defenses without consequences. Furthermore, he believed that excluding such evidence could undermine public confidence in judicial proceedings by allowing defendants to lie with impunity.