The Marine Corps is looking for a few good men, but it wants them "Semper Fi," not semper high.

To help keep the troops "always faithful," as the motto goes, officials at this sprawling, spit-and-polish base are cracking down on illicit drug use. In turn, they have drawn sporadic fire from some in the ranks who question urinalysis screening methods and alleged abuse of servicemen's rights.

The program, leading to punishment, demotion or even discharge from the Corps, is part of a military-wide campaign based on Defense Department figures showing widespread drug use and fueled by autopsy results showing drug traces in the blood of sailors killed last May in a crash on the aircraft carrier Nimitz.

Pentagon spokesmen say 138 soldiers in the Army's 82nd Airborne Division, screened in January before deployment to the peace-keeping force in the Mideast, tested positive for illegal drugs. The Navy recently discharged 12 sailors from its Orlando Naval Training Center as part of that service's get-tough policy.

"The Navy's taking the lead and the Marine Corps is going along with them," says Rep. James P. Addabbo (D-N.Y.), a frequent critic of drug use in the military.

Here alone, the antidrug offensive is massive. Since October, shortly before a new portable drug-screening kit went into use, nearly 10,000 Quantico marines have been urine-tested for drug use, says Maj. Barry Griffin, who manages the program. Of about 6,000 samples given further testing so far by a Navy laboratory in Portsmouth, 233, or 4 percent, were positive, Griffin says.

Sharply higher rates have been found among servicemen stopped for erratic driving on base or other offenses. Tests among those individuals have proved 31.9 percent positive, according to Griffin.

The Defense Department says a 1980 court ruling finding urine tests legal under certain circumstances, and a technological breakthrough in screening for marijuana use helped set the stage for the crackdown, which is now gaining full momentum in all services. The Navy's regional medical center in Portsmouth last month had a reported backlog of 15,000 urine samples.

None of which helps ease the anxieties of marines such as Gunnery Sgt. Clinton Morgan, who, despite 15 years in the service and five good conduct medals, has been whiling away his hours here lately. "I'm idling," Morgan says, "wasting my good talents."

"Gunny" Morgan found out recently the Marines may have other plans for him--like kicking him out of the Corps. Since January he has been assigned, without any duties, to a special unit of enlistees facing probable discharge. The others have been instructed not to talk to him.

The 35-year-old North Carolinian is one of the hundreds of marines snared by the drug-screening program and threatened with the possibility of job-hunting in the unemployment-plagued civilian world.

"I enjoy the Marine Corps, enjoy the challenges, but it's not much of a challenge at Quantico," complains 24-year-old Sgt. Melvin Christian of Washington, who is being shown the exit after coming up positive seven times out of 32 tests for marijuana use.

Marine Corps officials defend urinalysis and the specter of punishment or discharge for offenders as tough but necessary. Under guidelines effective last month, officers may be ousted after a single confirmed drug test. Noncommissioned officers generally have two chances and junior enlisted personnel three.

Officials also contend the war is being waged fairly. "Everybody is tested when their number comes up, including the base commanding general," says Griffin.

The Corps policy, as stated last December in a servicewide message from the commandant, puts it bluntly: "The distribution, possession or use of illegal drugs is not tolerated in the United States Marine Corps . . . Each instance of use of an illegal drug renders the individual marine unreliable, unfit for duty and a risk to the safety of fellow marines."

Morgan and others see the battle from a different vantage. They claim the crackdown is a rush to judgment, carried out in gung-ho Marine style with scant regard for servicemen's rights or the reality of drug use in the outside world.

"The Marines are making no distinction between use and abuse," says Alexandria lawyer Frederick W. Ford, an official of the Virginia chapter of the National Organization for the Reform of Marijuana Laws.

Several enlisted servicemen accused of using marijuana or cocaine complained in interviews the program has been haphazard and poorly explained by senior officers, has few clear guidelines, grants wide discretion to officers for recommending discharges, and relies on a urine testing system that is unreliable.

Some have sought out civilian lawyers. The ACLU in Northern Virginia is considering taking on one or two Quantico cases, which could become the first area court challenges of both the urinalysis system and constitutional guarantees of equal protection under the crackdown.

One of those cases is Morgan's. Analyzed on a field testing unit in mid-January, Morgan's urine was found to contain a derivative of THC, the active ingredient in marijuana. Morgan waived his right to a court-martial and took his chances with a mast, a nonjudicial proceeding.

Faced with the urine test results, Morgan acknowledged during a hearing he had experimented with marijuana out of curiosity. He was fined, given extra military duty, restricted to the base for 45 days and recommended for a demotion. He has also been told, he says, he will be separated from the Marines.

Meanwhile, Morgan's test sample was sent for confirmation to the Navy medical center in Portsmouth. Last month it came back negative, contradicting the field test result. Morgan appealed his punishment through channels, but was rebuffed because of his statement.

"I feel the commanders are using the guidelines for their own vindictive reasons," Morgan says. Maj. Griffin, the drug program manager, says he is aware of Morgan's situation but cannot discuss specifics. Except to say, "He would have been better off to say nothing."

"Take a person, a career marine, who decides, well, I made it this far, hold a pretty good rank and no trouble from the system," says Sgt. Lloyd G. Lewis of the Bronx, also awaiting discharge here for drug use. "Then the system turns and comes inside of a person's body. This is not a person being caught with possession or, you know, flaunting it around. According to his work section, the marine is okay."

It is the method for coming "inside a person's body"--the urinalysis--that has drawn the most sniping from disgruntled marines. Christian, a five-year veteran from the District, denies ever using drugs and insists his seven confirmed positive tests are false.

Not true, according to Griffin. While conceding that false positives are possible on the type of field kit used at Quantico (and ran as high as 50 percent in the early days last fall), he says false negatives are more prevalent. Test results that may mean the end of a marine's career are subjected to further, more sophisticated testing in Portsmouth, he adds.

"No test is ever foolproof on its own," agrees Dr. Richard Hawks, a senior researcher at the National Institute on Drug Abuse in Rockville. He says the Marines' field system backed up by more specific tests in Portsmouth is "a good way to go. A positive with both methods has a very high probability of being correct."

Still, he cautions, "You can never fully anticipate human error."

Griffin concedes there were numerous cases of intentional bottle-switching or diluting urine with water when the program began. One marine, Lance Cpl. James A. Pogue Jr. of the District, says he sometimes substituted urine from his girlfriend's 7-year-old child. And Griffin says one battalion here destroyed 140 sample bottles when it suspected tampering.

Now, however, Griffin says samples are taken one-on-one--that is, under visual supervision--to allay fears of a mix-up. "I took the test, and I watched like a hawk when the guy labeled the bottle," Griffin adds.

Since Feb. 1, the Marine Corps has permitted confirmed test results to be used as evidence in courts-martial, separation proceedings and in determining whether a serviceman's discharge will be characterized as honorable or otherwise. The latter can affect an ex-marine's veterans benefits.

Maj. Griffin says he doubts that illegal drugs are any less available on this 97-square mile base since the program began. "You could probably buy 'em in Lejeune Hall," says the silver-haired major, referring to the base's headquarters building.

The base perimeter is largely unfenced, including the town of Quantico (which the base surrounds) and an expanse of Potomac River waterfront. Military police conduct regular vehicle searches at the base's main gate, but are under orders to wave through civilians who must cross base property to reach the town.

At lunch time, a few erect, closely cropped marines stroll along the town streets, dropping into the barbershop or drugstore or pausing at the window of a novelty shop featuring Marine Corps T-shirts, decals and plaques.

"To err is human, to forgive divine," says one brass plaque prominently displayed. "Neither of which is Marine Corps policy."