East German physicist Alfred Zehe, a stocky man with blond hair who speaks soft, accented English, was the key to the successful conclusion of three years of difficult negotiations leading to Tuesday's swap of four Eastern bloc spies for 25 persons who have "been helpful" to the United States, according to informed U.S. sources.

The sources painted the picture of Zehe as a respected top scientist, recruited by his government's intelligence service for an ill-conceived espionage mission and then ruined -- captured and sentenced to eight years in an American prison without having acquired any worthwhile secrets.

"He seemed more of a prestige person than a master spy," said one informed source. "Maybe they feel a little worse for having gotten him into trouble than they do about some of the slobs they use."

"It's true he's the most valuable of the spies swapped by the U.S. , but he's not a master spy," said another source.

Zehe's Boston defense attorney, Harvey Silverglate, portrayed Zehe, 46, as an amateur coerced by his government to spy in return for travel privileges and to save his academic standing. But the FBI says its investigation showed he is a professional spy affiliated with East Germany's ministry of state security, which collects foreign intelligence.

In a dramatic exchange at high noon Tuesday on a Berlin bridge, the United States traded the physics professor, along with a Polish businessman, a Bulgarian trade official and a German grandmother -- all convicted or indicted for espionage -- for 25 persons from the Eastern bloc, a 6 for 1 swap.

The four spies were described by U.S. sources as valuable, high-level intelligence operatives. The East Germans said they were especially eager to gain the release of Zehe, according to Western sources involved in the swap.

The negotiations for the swap "blew up" in late 1984, according to a government source. At that time, the East bloc's best offer was 18 of its prisoners in exchange for the four, the source said. "For some reason," he said, "the deal got better for the Americans" by the time it was done.

There was speculation that the Russians had been waiting to see if the American legal system would let Zehe off before they agreed to the swap. Sources said they believed that the Russians would have had to agree to the deal. "It's possible that until that final piece was there, Zehe sentenced, that no kind of skilled negotiating could have pulled it off," said a source familiar with the negotiations.

Those traded by the United States, in addition to Zehe, were: Marian Zacharski, a Pole; Penyu B. Kostadinov, a Bulgarian; and Alice Mickelson, an East German.

They were a small fraction of what U.S. authorities say is a swelling tide of agents dispatched by Soviet and East European intelligence agencies to buy, steal or otherwise acquire the secrets of American military technology.

Three of the four Soviet-bloc spies released Tuesday had been turned in by American citizens who pretended to be beguiled and instead cooperated with U.S. authorities, an FBI official said.

The FBI and the Naval Investigative Service had conducted a two-year investigation that led to Zehe's arrest at a Boston gathering of the American Vacuum Society, a group of physical scientists, in November 1983.

"There are accusements that I must reject," the physicist said after his arrest. "A highly trained spy? I am very sorry, I am a university professor." Zehe later admitted that, under the direction of East German intelligence authorities, he had met a U.S. Navy employe seven times in 1982 and 1983 in Mexico City to buy secret documents. Affiliated with the university of Dresden in East Germany, Zehe had been an exchange scholar at the University of Puebla in Mexico since 1976.

Zehe paid the Navy employe $21,800 for documents concerning military technology, and also gave him a special camera, capable of taking as many as 2,600 still photographs on one film cassette, plus film that had been modified to photograph documents, the FBI said.

The Naval employe, who worked at the Naval Electronics Systems Engineering Center in Charleston, S.C., was working with U.S. authorities from the beginning. He had first discussed selling documents with Dieter Walsch, an attache at the East German Embassy here and an alleged intelligence agent, in 1981, the FBI said.

Zacharski, the most famous of the four in the United States, was sentenced to life in prison three and a half years ago, at age 30, for conspiring with a Hughes Aircraft Co. radar engineer, William Holden Bell, to have film of documents on key weapons systems delivered to Polish agents in Europe.

Posing as a Polish businessman, he had befriended Bell in the late 1970s at a fashionable condominium apartment complex in Los Angeles where they played tennis together and watched their children play in the pool, according to court records. He offered to assist Bell with some financial difficulties.

Bell eventually pleaded guilty to espionage, testifying that he agreed to provide documents to Zacharski in exchange for $95,000 in gold and cash. He allowed himself to be wired in order to record his dealings with Zacharski.

Bulgarian business official Penyu B. Kostadinov, 41, was arrested after a dinner at the Top of the Park restaurant in New York City, where the FBI said he was given documents relating to nuclear energy secrets by an American who was cooperating with U.S. authorities. Kostadinov was indicted in September 1983.

The Soviets selected East German Alice Mickelson, 67, a grandmother and a teacher of Marxism, as a spy courier "because they felt at her age no one would suspect her of such nefarious activities," a New York FBI official said afterMickelson was arrested last fall in New York City. Sentenced to five years probation, she was convicted of trying to smuggle classified American military data out of the U.S., concealed in a cigarette pack, to the KGB. She received confidential cables from a U.S. Army intelligence officer pretending to work for the Soviets, the FBI said.