The exclusionary rule dates from the 1914 Weeks decision and has come under increasing attack. In the recent case of United States v. Williams, the Court of Appeals for the Fifth Circuit held that a court should not exclude evidence where a police officer acts in the good faith but mistaken belief that the conduct conforms with the fourth amendment. State and pending Federal legislation endorses this approach. The Supreme Court has not adopted the good faith exception, although a majority apparently favors modifying the existing exclusionary rule. The pending case of Massachusetts v. Sheppard and two companion cases will offer an opportunity to modify the rule, however. The Sheppard case involved the police use of a defective warrant to obtain bloodstained clothing and other evidence linking Sheppard to a murder. Although the good faith exception could be used in this case, the harmless error approach would be better. It allows an appellate court to uphold a conviction where the admission of unconstitutionally seized evidence does not significantly prejudice the defendant's interests at trial. The Sheppard< warrant did not prejudice the defendant's rights, because the police undoubtedly had probable cause to support their warrant. Sheppard's objection was to the failure to state in the warrant the particular items to be seized. A harmless error rule in such situations educates the judge or magistrate on the constitutional significance of the mistake, whereas a good faith exception focuses solely on the police officer's conduct. The harmless error rule also permits use of evidence if the defendant's fourth amendment rights have not actually been harmed. Footnotes are supplied.