Convicted murderers trying to avoid the death penalty have a constitutional right to present evidence of their subsequent "good behavior" in prison to show why they should be spared, the Supreme Court ruled yesterday.

In an opinion by Justice Byron R. White, the court overturned the death sentence of Ronald DeRay Skipper, convicted of murder and rape in South Carolina in 1982.

The trial judge had refused to allow Skipper to present witnesses to show the jury that his life should be spared because he had "made a good adjustment" during his 7 1/2 months in jail between arrest and trial. The state supreme court agreed that such testimony would be "irrelevant and hence inadmissible."

But White, citing high court decisions in 1978 and 1982, said Skipper had a right to present "any and all relevant mitigating evidence that is available."

"Evidence that the defendant would not pose a danger if spared (but incarcerated) must be considered potentially mitigating," White said, and "such evidence may not be excluded from the [judge or jury's] consideration."

The ruling is not likely to have an impact on more than about 25 of the nearly 1,700 prisoners awaiting execution nationwide, according to attorney Jack Boger of the NAACP Legal Defense and Educational Fund Inc.

The ruling would directly affect four or five South Carolina inmates, Boger said, and perhaps as many as 20 from other states. But the decision clarifies earlier rulings, he said, that left unclear what evidence could be introduced to try to spare defendants from death row.

"It's going to make a difference in a number of cases" in the future, he said, because some states have tended to narrow what the defendant may prove to the jury as justification for life sentence. This appears to give broadest latitude" to those defendants and sends a signal to their lawyers, he said.

The ruling "changes the character of future sentencing hearings," Boger said, by giving the defendant a chance to present more evidence that might persuade a jury to allow him to live.

Justice Lewis F. Powell Jr., though agreeing with the result in this case, said the evidence Skipper wanted to present had nothing to do with deciding whether he should be sentenced to death. What mattered under prior high court decisions, Powell said, was a defendant's behavior before a crime, not afterward.

"In this case, for the first time, the court classifies as 'mitigating' conduct that occurred after the crime and after the accused has been charged," Powell said.

"One arrested for a capital crime, particularly a convicted defendant awaiting sentencing," Powell said, "has every incentive to behave flawlessly in prison if good behavior might cause the sentencing authority to spare his life."

"After today's decision, competent defense counsel in capital cases will instruct their clients to behave like Eagle Scouts while awaiting trial, and more particularly while awaiting sentencing," Powell said.

Powell, joined by Chief Justice Warren E. Burger and Justice William H. Rehnquist, said Skipper should have been allowed to present evidence about his conduct in jail but only because prosecutors presented his behavior there as a reason why he deserved to die.