In what circumstances will a prosecutor be allowed to refer to allegations of sexual assault against a witness?

In Laughlin v. United States, 128 U.S.App.D.C. 27, 385 F.2d 287 (1967), the prosecutor, while questioning a witness before the grand jury, made a reference to allegations of the defendant that another witness was a prostitute. The court noted that the record did not reveal when and even if the defendant had ever made such allegations and that the prosecutor's reference therefore "appears to have been injected gratuitously by (the prosecutor) in a way that could have prejudiced (the defendant) unfairly." Id. at 292.