The Supreme Court, in two rulings yesterday, completed a major narrowing this year of the protections afforded criminal defendants against illegal police searches.

In one decision, the justice ruled that eveidence seized illegally in one person's apartment could be used to convict a second person of possession of stolen mail.

The other decision held that drugs taken illegally from the purse of a woman could similarly be used to convict a friend of possessing the durgs.

The court already has held that only breach of a defendant's own rights -- not a breach of a bystander's rights -- mandates exclusion of evidence at trial.

Until yesterday, however, an exception was made for defendants charged with possession of illegal items. Those charged with posession, it was thought, would be unable to prove an illegal search without incriminating themselves.

The two decisions yesterday followed by two days an even broader ruling allowing the use in trial of evidence stolen by government agents from the briefcase of a nondefendant.

That decision also restricted the power of federal courts to throw out such cases as a deterrent measure, whether or not a defendant's own rights were violated.

All the case involve the exclusionary rule -- the controversial innovation by which past Supreme Courts attempted to deter illegal searches and confessions by barring their use as evidence.

The Burger court, though believing that the rule often thwarts the search for truth in criminal cases, has been unwilling to eliminate it. Instead, the court has chosen to confine its use to exceptional, clearcut circumstances.

The stolen mail case, U.S. vs. Ssalvucci, involved a police search of the apartment belonging to the defendant's mother. At the apartment, Massachusetts police found checks that were part of a cache of stolen mail and eventually charged John Salvucci and Joseph Zackular with possession.

The defendants tried to suppress the checks as evidence, charging that the search warrant used to invade the apartment was invalid. A federal district judge agreed.

But the prosecutors aruged that even if the warrant were invalid, it was direction not at the defendant but at his mother, who was not on trial.

In a 7-to-i decision, Justice William Rehnquist wrote for the majority that the old ruld exempting possession offenses has been rendered obsolete by other Supreme Court rulings that protect defendants from self-incrimination.

The exception "now serves only to afford a windfall to defendants whose Fourth Amendment rights have not been violated."

The second case, Rawlings vs. Kentucky, involved a police search of a purse in which illegal drugs had been stashed.

The woman who owned the purse emptied its contents onto a table during a police search of a house and told David Rawlings, a defendant, to "take what was his," including the drugs. He then admitted ownership of the illegal drugs.

Prosecutors used this statement at Rawlings' trial and won a conviction. Rawlings claimed that search of the woman's purse was illegal. He also argued that police had illegally detained him and his companions during the search by holding them "until they got a search warrant."

But the high court ruled 7 to 2 against Rawlings.Justice Rehnquist said Rawlings had no "legitimate expectation of privacy" in the woman's purse and thus his rights were not violated.

In addition, the court said, his admission of ownership of the drugs nulified any contention that the confession was prompted by the illegal detention.

In other rulings yesterday:

The court invalidated the Texas death penalty statute because it allows dismissal of jurors who express opposition to capital punishment. That provision prejudices the defendant's right to a fair trial, the justices ruled, 8 to 1, in Adams vs. Texas.

The court ruled that the uavailability of a witness for cross-examination at a stoled credit card trial in Ohio did not invalidate the conviction. The court ruled, 6 to 3, that when "hearsay" testimony is involved, a transcript version of the testimony, if reliable, is adequate.