ONLY 55 PER CENT of American adults exercise, according to a survey by the President's Council on Physical Fitness and Sports, but 57 per cent say they get enough exercise.

Paradoxically, inactive Americans are more inclined to think they get enough exercise than active ones. Sixty per cent of the 49 million who do no formal exercising said they exercise enough; only 53 per cent of the active group said the same.

What can be done? Government, as the country's third-largest employer after manufacturing and sales, might consider on a grand scale the employee fitness programs U.S. corporations have sponsored for years.

Some fitness leaders say the government is reluctant to push programs. The rationale: the typical taxpayer, typically unfit, remains unsold on the benefits of fitness and would balk at the idea of government workers taking time off to prance around a gym - unless, of course, those workers were FBI agents or astronauts.

Never mind that the coins one drops into a Xerox machine help keep that company's employees in sneakers. When it's tax money it's different.

Whatever the reason, government lags far behind industry in trying to trim losses by trimming employees silhouettes.

A PCPFS slogan is "good health means good business," NASA has its Division of Occupational Medicine, the Pentagon has its Officers Athletic Association and there are gyms at Interior and State. But the only government employee fitness program that approaches the best of the corporations' in scope and method is the occupational health physical fitness program at the Justice Department.

On a recent weekday morning the small gym in the basement of Justice was crowded. A man with glistening biceps the size of watermelons was pumping 240 pounds of iron on a circuit trainer. Two others - red-faced and dripping sweat - were jumping rope at a furious rate.

Ten more were warming up with shoulder shrugs while group leader Bert Knitter counted and shrugged along.

The 10 were taking part in the employee fitness program initiated at Justice last August for those, unlike the man with the big biceps, not independently motivated to keep fit.

After the warmups the group exercised vigorously for 10, 7-and 5-minute intervals. They bench-stepped, rode stationary bikes, rowed or otherwise kept in motion to tunes from a cassette deck. Between times they measured heart rates to see if they were overexerting or underexerting themselves. At the end of the hour they lay on the floor and relaxed.

How did this pioneer program come about?

When the FBI moved out of its old headquarters two years ago its exercise room was vacated. Edward Scott, deputy assistant attorney general for administration, began making moves on the space. He asked Larry Bogatz, a planning and development specialist, to work out logistics, and Knitter, formerly with the State's recreation association, to administrative.

Three things make the program they devised unique in government. Employees from Attorney General Griffin Bell on down to GS5s, are eligible (for this preliminary round about 300 from all levels were chosen); it is run during regular workday hours, with each participant getting three hours of administrative leave per week for workouts; and it is free - Justice pays the $365-per-person bill.

Before anyone was admitted he was cleared by a doctor. Next the group underwent stress testing conducted with some of the most sophisticated equipment in the area.

So far 315 persons, about 28 per cent women, have been tested. Of those, about 50 showed some symptoms of cardiac disease. The greatest risks were screened out, as were heart attack victims.

"This isn't a cardiac rehab program," said Paul O. Davis, who directs the testing.

"Our focus is health, not sickness," said Georgetown University nursing student Linda Tyson, who with four other volunteer seniors joined the staff in February.

Is the program a success? Participants say yes. Roland W. Finken, 46, head of a small training unit at Justice, said in six months the results of his stress tests have improved 30 per cent.

"Recently there were a couple fo times, especially with the reorganization, that I could have blown my cool," he said. "But I didn't."

Dave Hunter, 35, an attorney with the civil rights division, also is pleased. "The government is losing some of my time but I think I make it up in other ways. "He said initial tests showed his flexibility was poor, his strength mediocre; he could only come within six inches of touching his toes. Now he can touch his sneaker tops.

"And I just did 45 situps. Six months ago I could only do 28."

The program cost $500,000 this year, which covered initial expenditures for equipment and construction of a still-unfinished second exercise room and locker facility. Next year, Knitter said, costs will be about $200,000.

Where will the program go from here? Knitter hopes eventually to involve as many as 1,000 of Justice's 7,500 employees.

What of those who may need the program more than others but won't bother?

"Some of these guys are walking time bombs," said Davis. "And they'd best clean up their acts or take the consequences. But how do we motivate them? We aren't trying. We are, however, going to use the CPI (California Psychological Inventory) to find the people who will stay with the program after initial testing and who won't waste our time."

Since the program began, about 35 people have dropped out. Some changed jobs or were transferred. The others couldn't find the time.

That excuse isn't good enough at some places, Knitter pointed out. "At the Tyler Tool Co. in Texas, where they have an excellent fitness program, if you can't organize your time well enough to find an hour or two for exercise, then they don't want you."