Stephen M. Bingham, who lived underground for 13 years, was acquitted today of charges that he smuggled a pistol to inmate-author George Jackson that led to the 1971 San Quentin Prison massacre in which Jackson and five others died.

Bingham, who came out of hiding in 1984 to face murder and conspiracy charges, rose from the defense table to hug his wife and his father, former Connecticut state legislator Alfred Bingham.

Bingham's supporters, many of whom sat through the trial and waited out the jury deliberations, gasped. Some began crying.

Now 44 and gray-haired, Bingham fought back tears as he emerged from the courtroom and told reporters in a shaking voice, "I've waited 15 years for this day."

Bingham was 29 on Aug. 21, 1971, when he visited Jackson at San Quentin. Later that day, Jackson, two inmate trusties and three prison guards died in a carnage that prison authorities said began when Jackson fired at guards with a 9-mm semiautomatic pistol smuggled to him by Bingham.

Bingham was charged with one count of conspiracy and two counts of murder in the deaths of prison guards Frank DeLeon and Jere Graham, who were killed with the smuggled pistol.

Bingham was a Berkeley lawyer whose activity and radical causes included working with Jackson, a Black Panther Party leader known for his published letters from prison, on a possible suit against the California Department of Corrections.

Bingham testified during his 2 1/2-month trial that he went to the prison only to deliver some edited manuscript pages to Jackson. He was accompanied that day by Vanita Anderson, a Black Panther aide who had exhausted her visiting privileges.

Anderson gave Bingham a briefcase which he passed to Jackson. The government contended that Bingham knew the case contained the gun that Jackson later pulled on guards; Bingham testified that he never looked inside the case and had no knowledge of the gun.

Bingham said in his testimony that he changed his name and fled the country because he was "in total terror" of being framed on murder charges. For most of the next 13 years, until he surrendered to Marin County authorities in July 1984, he lived in Paris and worked as a house painter.

In a recent interview with The Washington Post, Bingham said he decided to give himself up because he thought "the moral climate in this country" had changed enough to afford him a fair trial.

The jury of 10 women and two men had deliberated 23 hours over five days. The jurors said they had agreed among themselves not to discuss the case, and left the Marin County courthouse, escorted by two bailiffs, without talking to reporters.

The prevailing opinion among observers who waited out the verdict held that, given the prosecution's largely circumstantial evidence, it would be difficult to find Bingham guilty beyond a reasonable doubt.

Bingham's attorney, M. Gerald Schwartzbach, praised the jury and Superior Court Judge E. Warren Maguire, added, "Thank God this nightmare is over."

"This case represents something much larger and more important than myself," Bingham said of the verdict.

Bingham said he believes that, "in 1986, the values of the '60s endure." As evidence of that, he pointed to the opposition to aid for Nicaraguan counterrevolutionaries and the movement to fight South Africa's apartheid laws.

Bingham said he does not have any definite plans. "This is the first day of the rest of my life." But he added that he would "probably" practice law. Bingham's father paid his son's dues to the California State Bar during the younger Bingham's fugitive years.

A member of a prominent New England family, Bingham married a French woman, Francoise Blousseau, 25, in 1984. Blousseau said she and her husband do not know where they will live. "We did not want to make plans until this was over," she said. "We did not want to be deceived."

A friend of Bingham from his law school days at Berkeley's Boalt Hall, Peter Haberfeld, overheard Blousseau and said, "A lot of friends are going to persuade them to live in California."