Plea bargains and immunity agreements are contracts between an individual and the Government in which the Government offers an inducement, such as a reduced charge or immunity from prosecution, in return for a guilty plea or testimony. When testimony is the object of the agreement, the witness must agree to testify truthfully; this truthfulness agreement becomes part of the plea bargaining arrangement. When a prosecutor tells a jury that a witness's truthfulness agreement makes the witness more believable, the prosecutor is making a statement that is functionally equivalent to vouching for the witness's credibility. Although many courts have expressed concern about the abuses of prosecutorial vouching, they have generally failed to address the problem adequately. The harmless error doctrine permits courts to affirm convictions based upon testimony which is endorsed by the prosecution despite questionable prosecutorial conduct. Judicial reluctance to insist upon a universal requirement of reversal, even when vouching or other prosecutorial misconduct is found to be prejudicial to a defendant's case, is the final obstacle to the effective control of the misuse of truthfulness agreements. The tone of appellate court warnings about the practice in such cases as United States v. Spain indicates that courts would welcome a solution to the problem. The article suggests that truthfulness agreements should be excluded from evidence and that a per se rule of reversibility should be invoked when a prosecutor uses such an agreement to bolster the credibility of a witness. Thus, the proposed exclusionary rule would eliminate misleading and improper prosecutorial influence on the jury. The note provides 91 footnotes.