For a decade, DNA tests have been the most powerful tool available to police and prosecutors investigating new crimes, helping to pinpoint suspects in rape and murder cases, while alerting authorities when they were on the wrong track.

But the tests have created an unprecedented dilemma, both locally and nationally: How and when should the justice system reach into the past and apply the test to critical evidence in criminal cases long closed by a conviction?

As of now, no inmate in the prison systems of Maryland, Virginia or the District has an absolute right to have evidence resurrected and tested to see whether a conviction was just. Instead, prosecutors, judges and elected officials have been deciding on a case-by-case basis which prisoner pleas to grant, as Virginia Gov. James S. Gilmore III (R) did last week when he ordered DNA tests for Derek R. Barnabei just days before his scheduled execution for rape and murder.

But a debate has erupted about whether fairness demands that DNA tests be done automatically for any incarcerated felon who can reasonably show it could help prove his innocence. The debate's urgency arises from the fact that more than 70 people have been freed from the nation's prisons--including eight from death row--after they sought and got DNA tests that proved they were convicted wrongfully.

Given those numbers, some jurisdictions are choosing to err on the side of generosity. Illinois and New York now guarantee prisoners access to new testing if certain criteria are met, and San Diego prosecutors have gone a step further: They're combing through old files to see whether the state should order and pay for new laboratory work.

"We don't want innocent people in jail. It's not much more complicated than that," said Anne Arundel County State's Attorney Frank R. Weathersbee, who last year allowed new testing for Albert G. Givens, who had been convicted of a 1992 murder and sexual assault partly on the strength of a more primitive DNA test. The new test helped confirm Givens's guilt.

Others argue that DNA testing, which compares samples of genetic material found at a crime scene with samples from a suspect, is expensive and that reopening old cases upsets victims and diverts resources needed to solve new cases. Court and police personnel worry about the logistics of saving evidence for years on the off chance that still more advanced DNA testing could someday make it worth examining.

Besides, they note, post-conviction DNA testing has implicated, rather than exonerated, 50 percent to 60 percent of the inmates who have been granted analysis.

"I've allowed it four or five times, and I've yet to have one come back where the guy didn't do it. Invariably, they turn up guilty," said Fairfax County Commonwealth's Attorney Robert F. Horan Jr., who has gone to court to fight a testing request that he considers frivolous.

In the absence of any mandate that they must test, the practice of prosecutors and judges in the region varies. Generally speaking, District and Maryland officials look more favorably on requests for post-conviction DNA testing than those in Virginia, where inmates are also hampered by a state law that gives them only 21 days after their conviction to ask for a new trial based on new evidence.

In other words, whether an inmate gets DNA testing largely depends on where the crime was committed. And that, some experts and politicians say, is unfair.

"Many states have no clear process and continue to resist legitimate requests for testing. There is a clear need for a minimum national base line," said Sen. Patrick J. Leahy (D-Vt.). "No state should deny the access to truth and justice that DNA offers."

Leahy has introduced a bill that would mandate DNA testing for a wide variety of inmates who seek it and require authorities to notify prisoners before disposing of the evidence from their cases. His bill has drawn strong support from civil liberties and prisoners' rights groups--and Kirk Bloodsworth.

A Cambridge, Md., waterman, Bloodsworth was sentenced to death--twice--for the 1984 rape and murder of a Baltimore County third-grader. In prison, he read a book about the first British case based on DNA testing. Eventually, prosecutors agreed to test a dime-size spot of semen found on the child's underwear, and the results excluded Bloodsworth.

"We should make every effort not to kill an innocent person," said Bloodsworth, 39, who walked out of prison June 28, 1993. "If a man is sitting on death row or in prison for 10 years or 20, and there's something that could help him prove he was innocent, he should be allowed to do it."

But many prosecutors, court officials and politicians say such blanket guarantees would overwhelm laboratories and courts with frivolous claims. They support a Republican counterproposal that would limit tests to those who were convicted before DNA technology existed and who could show that a test alone would prove their innocence.

"People say, 'Why not give everyone a DNA test? What can the harm possibly be?' " said Joshua Marquis, a board member of the National District Attorneys Association. "It's an enormous waste of resources, and it reopens the wounds of the real victims. . . . If the DNA labs are doing ridiculous claims, there is another DNA test they are not doing."

Marquis and local advocates of limited testing point to cases like that of Alexander Cameron. In 1996, an advocacy group called the Innocence Project sought DNA testing for Cameron, who was serving two life sentences plus 25 years for an Alexandria rape and robbery. Prosecutors were skeptical, because he had once written a letter to a judge that seemed to apologize for the crime, but they eventually allowed the testing.

When the results came back, Cameron was a match, court records show. The Innocence Project dropped his case, although he is still pursuing additional tests.

"Some people really don't understand DNA, or they think they have nothing to lose or maybe he can't bear to tell his mother he did it," said Jane Siegel Greene, the project's executive director.

The Innocence Project has at least 22 requests for new DNA testing pending in Virginia, Maryland and the District. Project lawyers recently persuaded Gilmore to order new testing in the case of Earl Washington Jr., who has served 17 years in prison for a rape and murder. They have also filed suit against Fairfax County on behalf of James Harvey, convicted in 1990 of raping a woman who was abducted by two men as she walked down Route 1.

Horan says he is fighting the request because the other evidence against Harvey is strong. He argues that a DNA test that did not match would not necessarily clear Harvey in the crime, because there were two assailants.

Inmate advocates have had better luck in the District, where prosecutors have a general policy to allow testing even in cases where they feel sure the jury got it right, said Channing Phillips, a spokesman for the U.S. attorney's office. And Maryland's courts and prosecutors have been more generous than many other states in allowing testing, particularly when judges are concerned that the original case was weak, advocates for DNA testing say.

"If there was no other physical evidence linking the defendant to the scene . . . I'd be inclined to order testing," said Prince George's Circuit Court Judge William B. Spellbring Jr., who has never received such a request. "But if there's fingerprints and 36 eyewitnesses, then we're wasting resources."

Locally, only five of the 15 prosecutors' offices--Fairfax, Prince William and Anne Arundel counties, Alexandria and the District--say they have received requests from an inmate for new DNA tests. The results have exonerated men in Alexandria and the District.

Prosecutors in Arlington and Calvert counties have helped free inmates after DNA testing requested by the state--rather than the defendants--suggested they were not guilty of crimes they had confessed to. In both cases, other suspects surfaced and DNA helped confirm their guilt. Justice Department officials said they do not know of any post-conviction DNA testing requests from local inmates in federal prison.

For some inmates, locally and elsewhere, there is another problem beyond getting official permission for new lab analysis: finding something to test. Two weeks ago, evidence in Barnabei's case briefly disappeared from a locked vault in the Norfolk Circuit Court. Less dramatically, some area police and sheriffs departments routinely throw out biological evidence after the main round of appeals has finished, often within five to seven years of the conviction.

While Calvert County State's Attorney Robert Riddle advises law enforcement officials to keep the physical evidence from major cases indefinitely, Fauquier Sheriff Joseph Higgs Jr. said his officers routinely burn biological evidence once the inmate has completed his appeals, about five to seven years in non-capital cases.

In the District, the U.S. attorney's office recently had to turn down a request for DNA testing from Ervin Alston because the evidence from his 1978 rape case had been destroyed, Phillips said.

In the end, some academics say, the debate over who should receive post-conviction DNA testing is simply the tip of the iceberg. As scientists develop more sophisticated ways to test for the presence or absence of a particular person at a crime scene, more and more inmates will seek to challenge their convictions.

"What we ought to be noting right now is that our fact-finding mechanisms are lousy," said University of Wisconsin law professor Michael E. Smith, who sits on the National Commission on the Future of DNA Evidence. "The basis for a conviction in the first place is much less certain than legally and psychologically we assume it to be. We have been sloppy."

Edward W. Honaker agrees. He spent nearly a decade in Virginia prisons trying to persuade authorities that a rape victim and a witness had both been wrong when they told a jury he was the guilty one.

Finally, the Innocence Project and investigators from Centurion Ministries cast enough doubt on his conviction that state officials agreed to order DNA testing. In 1994, he walked out of Nottaway prison a free man.

"It's mind-boggling what our system can do. . . . What happened to me can happen to any man alive," said Honaker, 50. "DNA was my salvation."