It looked as if RKO-General was home free with the license renewal for its multi-million-dollar Boston TV station. It had won the first round before a Federal Communications Commission judge, and by all odds its license was secure.

But the challengers, Community Broadcasting of Boston, decided in 1974 to try one last shot - an investigation to see if there were any skeletons in the closet RKO-General's parent corporation, General Tire & Rubber Co., that would make it unfit under FCC rules to hold a TV license.

For this unusual approach to get a TV license, the challengers hired Terry Lenzner, who was starting a new legal practice in Washington after serving as deputy counsel of the Senate Watergate committee.

The Boston investigatio launched Lenzer into a unique legal specially practicd by few if any other lawyers in the country - the legal equivalent of an investigative reporter, a special prosecuter for hire.

He is a partner in a traditional Washington law firm, but the cases he brings in are anything but the prosaic matters of taxes, legislation, antitrust and administrative regulations that make up the bread and butter of legal work here.

"I start with nothing and see if there is a case there," said Lenzne, who was reluctant to talk about any cases that are not on the public record.

In the Boston case, for example, Lenzner traveled 25,000 miles to three continents to dig up enough dirt on General Tire - including the payment of bribes to Chilean leftists and Arab money moguls - to provide grounds for the Boston challengers to ask for a new hearing.

While the information he gathered does not guarantee that Community Broadcasting will wrest the license from RKO-General, it provided the challengers with their only chance for another shot at it.

"We started from an absolute standing start of sero," recalled David G. Mugar, president of Community Broadcasting who hired Lenzner on the recommendation of a mutual friend.

"The odds are still in General Tire's favor today. This is a very valuable properly, and the FCC can't be expected just to give it to us."

Nonetheless, the investigation uncovered irregulaities that for the first time are being used as grounds to try to strip a TV license from a corporation. They are that illegal oversea payments show the corporation lacks the moral standards neccessary to own a TV or radio license.

More than that, the information gathered by Lenzner has triggered investigations by the Securities and Exchange Commission, the Federal Trade Commission and a federal grand jury that go far beyond General Tire's fitness to hold TV licenses.

And, if it loses the Boston case, its 15 other radio and TV licenses are in jeopardy.

Since taking the Boston TV challenge, Lenzner has been hired by the Alaska Pipeline Commission and the State of Alaska to investigate cost overruns on the Alaska pipeline and has served as special counsel to two New York State investigate committees.

Other Washington lawyers call Lenzner a "spitbalier" because he tries to win cases by digging up dirt on behalf of his clients.

Lenzner, however, sees his practice as "a creative kind of works" and said other lawyers in his firm. Wald, Harkramer & Ross, like to be involved in his cases because they are so different.

"I find it a fascinating way to practice law," said Lenzner, who originally thought he would be spending most of his time in court as a trial attorney.

It also can be lucrative to the firm. The state of Alaska appropriated $800,000 for its legal services this year - including expenses and the salaries of more than 22 investigators, legal aides and lawyers on the case.

Mugar estimated Community Broadcasting's legal costs are about $250,000 - including the expenses of sending Lenzner and investigators around the world in search of evidence of overseas corporate bribes.

"It was worth every penny of it," said Mugar, "I don't think our case had a chance without it."

Avrum Gross, Alaska's attorney general, said the legal fees could run more than $1 million "by the time we get through, but the stakes are enormous."

Since finding overcharges in the construction of the pipeline means Alaska gets more money in taxes and royalties, the state could earn at least $130 million a year if Lenzner's projections before the Interstate Commerce Commission hold, he said.

"Every time the cost goes up, the state revenues go down," said Lenzer.

In Alaska, Lenzner and his staff are plowing through 150,000 documents looking for evidence that the builders of the pipeline added unnecessarily to its cost. In Washington, he represents the state before the ICC and is supplying information for investigations being run by the Internal Revenue Service and the General Accounting Office.

Gross said Alaska hired Lenzner because "our office simply did not have the resources," which were also beyond the capability of most private law firms in Alaska. As it is, four assistant attorney generals are working with Lenzner.

Lenzner's firm has opened a branch office in Alaska for this case. Most of the work is being done by three investigators - including one who came from the staff of the Watergate committee - and legal aides who do routine work under the supervision of a lawyer.

Lenzner, 37, is a former Harvard University football captain who started as an investigative lawyer in his first job - working for Dohn Doar in the Civil Rights Division of the Justice Department. His first assignment was the investigation of the murder of three civil rights workers in Neshoba County, Miss.

Later, he became the controversial director of the Office of Equal Opportunity's office supplying legal services to the poor. He was fired in 1970 during the Nixon administration.

Lenzner is one of three people who tied for sixth place in President Carter's selection committee's hunt for a new FBI director. Although his name is still mentioned as a long shot, Lenzner said he hasn't talked to anyone for months about the post.