When Warren P. "Don" Johnson decided it was time to leave the military after 35 years, he wasn't sure what his chances were for a job in the private sector.

"I had a lot of doubts about the marketability of my skills," he said in an interview. He had risen to the rank of lieutenant general in the Air Force, and served as director of the Defense Nuclear Agency in the years since he joined the Air Force as an 18-year-old enlistee.

But the military was his life, and he didn't know if that kind of background was acceptable to firms "on the oustide."

So he went to Jim Benton, founder and president of Janus Consultants, Inc., a Washington-based executive placement, or "headhunting," firm. And Johnson learned what many Washington bureaucrats and businessmen are learning. There is a rapidly growing lucrative executive search market in Washington.

Benton told the 55-year-old Johnson that he was in fact, an attractive hire. "One of the best things Jim Benton did for me was to give me the confidence I needed in my marketability," Johnson said.

In a short time, "before I even sent out my resume," Johnson was juggling at least five solid offers. He is now the vice president for personnel of Baxter Travenol Laboratories, a large firm based outside of Chicago. And he is earning "a major increase" in salary from his days in the military.

Johnson's case is more the rule than the exception in Washington these days. Besides a burgeoning market for executive search firms here, many New York-based headhunters are opening up Washington offices.

There are many reasons for the trend toward hiring out of the government. Because of increased federal regulation, many firms are hiring lawyers from the agencies they have to deal with - in order to take advantage of the insider's knowledge of the workings of the complex bureaucracy.

Top-flight managers in the government still don't command the kind of salaries that their counterparts in private industry draw, making the public servants "a good buy." And Washington offers an unusually large number of people with advanced education degrees.

In addition, there are attractive pools of talent with training in such specialized areas as computer technology. And more and more companies are making Washington their corporate headquarters, creating a large private-sector market here as well. And the number of trade associations that have set up in Washington has increased exponentially.

One of the few actual barometers of change in the Washington executive market has been the increased in "Professional Opportunity" advertising at the Washington Post. Until a year and a half ago, there was no such special catagory advertising which offers executive-level placement, in the Post.

But the growth of such advertising since then has been astronomical, to the point where two weeks ago there were more than six pages of such advertising in the Sunday Business & Finance section. According to Maryanne Neiderhauser, who is in charge of professional opportunities ads in the Post, only New York, Chicago and Los Angeles newspapers traditionally have had significant advertising of that type.

William E. "Wes" Simmons heads Simmons Associates, a leading executive search firm in Washington.

"One of the reason's we're in D.C. is because we can tap the government marketplace," says Simmons. "In doing a general counsel search for a billion-dollar energy corporation, we're asked to consider candidates that have a government background as well as private-industry people in the energy field.

"There is an advantage to getting someone who knows government," adds Simmons, "because more and more of industry is coming under government regulation."

Simmons also said that there has been a growing demand for the services of executive search firms by local companies. "Five years ago, 20 per cent of our business came from local firms, now it is 50 per cent. They have realized that a search firm can give them something very important. We can identify good people who aren't looking for a job, but may be available."

Shirley Lamdan and William Basist opened their executive search and consulting business (Basist-Lamdan) here a year ago. "A lot of companies realize that there are a lot of good people in government," says Basist. And there are a tremendous number of Fortune 500 companies that are working on government projects and need the kind of specialized expertise in dealing with the government that can only be found in people who have worked there."

"We tell people in government what they have to offer," said Lamdan. She said her firm sponsors seminars for government workers thinking about going into the private sector. "We alert them to the advantages and disadvantages of going into the profit settings."

Basist said his rule of thumb for expected salary increase after leaving the government is "about 20 to 25 per cent."

But he adds that the Washington market is not just Washington. "This is a region that starts in Harrisburg, Pa., and goes all the way to Richmond. And the non-government part of the economy is growing rapidly." He points to increase commitments to facilities in the Washington area from Mobil, Time-Life and several retail chains.

Milton Haberman is the president of Interquest, Inc., a New York firm founded in 1973 that specializes in recruiting lawyers for corporations. He says there has been a surge in the hiring of in-house lawyers for large corporations.

"We use Washington as a source of good, qualified people with government experience in the specialty fields we serve," says Haberman. "We are always looking at lawyers working in antitrust, or with agencies like the Federal Trade Commission, Consumer Product Safety Comission, Internal Revenue Service."

He says two factors have enhanced the market for government lawyers looking to private practice. "There is an increased amount of regulation, and there is an increased demand for in-house lawyers.Rather than going to a Washington law firm, corporations are hiring their own counsels," he said.

But, Haberman warns, "If a lawyer has been with the government too long, most companies will shy away. If the lawyer has been in an agency for from one to five years, he or she is still considered salvagable. After six, seven or eight years, they are considered lobotomized in terms of ability to function in a corporation. Their orientation is good-guy, bad-guy, and they frequently can't see another point of view." He admits that there are exceptions.

There are, for example, senior government people who run departments, or sections of departments, who are "extremely capable," Haberman says. "If a young lawyer jumps from the government, he may get a 25 per cent increased in salary, although the government is paying its young lawyers much better these days. But for a senior guy, who is in charge of a section, the sky is the limit."

He points to a Labor Department official who at age 30 left to work for a Washington firm and doubled his salary to more than $100,000.

All of the executive search firm owners interviewed said it appears that government employees are beginning to shake the notion that they are not profit oriented and thus unable to function in the private sector, despite a recent survey from Simmons Associates that disclosed that former Nixon-Ford administration high-level apointees were having difficulty getting jobs in the "real world."

"That," said one of Simmons' competitors, "may have something to do with that particular administration. They didn't have the best record for running the government, you know."