Reputation or opinion is admissible to impeach a witness by showing the untruthfulness of the witness.

Crawford v. Washington, 541 U.S. 36, 68 (2004) ("Whatever else the term ['testimonial'] covers, it applies at a minimum to prior testimony at a preliminary hearing, before a grand jury, or at a former trial").The U.S. Supreme Court and other courts have said that statements made in formal judicial proceedings are "testimonial" and are therefore subject to the constraints imposed on the use of testimonial hearsay in criminal trials by the Confrontation Clause of the Sixth Amendment to the U.S. Constitution.

City of Tuscaloosa v. Harcros Chemicals, Inc., 158 F.3d 548, 557-558 (11th Cir. 1998) (holding, in plaintiff Alabama municipal entities' action against defendant chemical companies for fraud and violation of antitrust law, that the trial court....A statement is imputed to a principal or to an employer and is treated as the principal's or employer's vicarious admission (which, like other admissions is exempt from the hearsay rule) if the principal or employer is a party, the statement is offered against the principal or party, the statement is a statement by the party's agent or employee about a matter within the scope of the agent's agency or the employee's employment, and the statement is made during the principal-agent or employer-employee relationship.

Fed. R. Evid. 608(b) (specific instances of the conduct of a witness "if probative of truthfulness or untruthfulness" may be inquired into during the cross-examination of the witness).Prior acts of a witness may be elicited during the interrogation of the witness to impeach the truthfulness of a witness, but the acts so inquired into must in fact be probative of truthfulness.