text: use of illegally obtained evidence for impeachment purposes?   even though the prosecution in its case in chief can not use evidence illegally seized to convict the defendant, the evidence may be used to impeach the credibility of the testimony given on direct or cross examination.  u.s. v. havens, 446 u.s. 620 (1980).  however, the evidence can be used to impeach defendant's testimony, not one of defendant's witnessess.    the exclusionary rule (rule excluding illegally obtained evidence) only applies in criminal proceedings.