There is a new reality for millions of American workers, and Juanita M. Jones, a 49-year-old grandmother of four, contends that she is a victim of it.

A District school bus aide for disabled children, Jones, who had been praised as a model employe, was fired in August 1984 based on a single urine test that indicated marijuana use.

Jones, who has sued the city in federal court, was tested as part of a mass screening of school transportation workers. Although she denied using drugs and supplied two additional urine tests to support her claim, she was told to punch out her time card and was fired immediately.

Jones is caught up in a phenomenon affecting thousands of American work places as employes and job applicants are being required to let their body chemistries reveal their personal secrets.

Professional athletes, police officers and ordinary office workers are submitting urine samples to be checked for evidence of marijuana, cocaine and other drug use. Thousands of military recruits -- and soon the entire 2.1 million active duty force -- are having their blood tested for exposure to the AIDS virus. Bus drivers and amusement ride operators are being told to spit into plastic cups to have their saliva examined to see if they have recently smoked marijuana.

Mass screening to detect a variety of drugs -- sometimes weeks after use -- has become economical and popular. At the same time, technological advances have made it possible to test people for susceptibility to an array of diseases, from AIDS to sickle cell anemia. Although genetic screening is not thought to be performed widely, many observers say it is just a matter of time before such tests are perfected and put to use.

Employers praise drug testing as necessary and prudent in an age of widespread drug abuse, which they say cuts into productivity, increases absenteeism and health costs, and poses the threat of lawsuits by injured coworkers and customers.

"If somebody smokes pot on a Saturday night, it's the employer's business on Monday," said Peter B. Bensinger, former director of the U.S. Drug Enforcement Administration and now a consultant to businesses on drug testing. "It is the company's problem if its absentee rate is 2 1/2 times higher, its accident rate is 3 times higher and the medical costs are out of sight" because of drug abuse.

Many workers find the tests embarrassing, intrusive and unfair. "What you do on a vacation or weekend should be your own business," said bus driver Gerald Dial, 35, of Greenbelt.

Dial was fired by the Washington Metropolitan Area Transit Authority after his urine tested positive for marijuana after a bus accident in 1983. He was rehired a year later after he won a grievance settlement "because of some evidence problems with the sample," according to Bob Kniaz, an attorney for WMATA.

"I never considered myself a get-high kind of guy, but smoking a joint while off the job was my form of relaxation, like a martini," said Dial. "But you can't do anything now. You're paranoid all the time you'll be tested."

According to a recent survey, at least 25 percent of all Fortune 500 companies screen for drugs in pre-employment physicals, up from 10 percent in 1981. Bensinger estimates that half the Fortune 500 companies will institute testing programs by 1988.

"With polygraphs, it was the sleazy companies," said Robert Ellis Smith, editor of Privacy Journal. "Now it's the Fortune 500. Even IBM is using it."

Opponents of the tests contend that they delve into the personal lives of employes who should instead be measured by on-the-job performance. More important, they warn, the tests are frequently unreliable, presenting the risk that an employe who has not used drugs will be unfairly dismissed as a result.

"There are a lot of things that may affect absenteeism and health and productivity that have nothing to do with drugs," said Arthur B. Spitzer, legal director of the American Civil Liberties Union's Washington office. "I think we would all feel very resentful if our employer inquired if we were staying up at night watching the late movie or having an extramarital affair."

Some critics of the tests say they are better suited for employes whose jobs involve public safety.

"What may be appropriate for the airline pilot may not be appropriate for the ticket clerk," said San Francisco attorney Kathleen Lucas-Wallace, who is representing a woman fired for refusing to take a urine test. "If the ticket clerk is writing tickets in a satisfactory manner, why should they be tested? If not, why don't they get fired for poor performance?"

While "nobody wants to get on a plane with a drunk pilot," she said, "it's not been a tenet in American society that employes are slaves of employers."

But Mark A. de Bernardo, manager of labor law for the U.S. Chamber of Commerce, turns these concerns aside. "The innocent have nothing to fear with it," he said.

Whatever its legitimacy, drug testing by American employers has exploded in the last two years as medical companies saw a mass market for the tests and began to sell them aggressively.

"The work force now is made up of people who went to college in the '60s and have taken their drug habits with them," said Mark A. Rothstein, a professor at the University of Houston Law Center who has written extensively on drug testing. As a result, he said, testing "is big business."

That business now reaches into nearly every segment of the work force, affecting white-and blue-collar workers alike:

*The nation's 14,000 air traffic controllers will soon be required to submit to annual drug tests, under a program approved last year by the Federal Aviation Administration. Similarly, the Drug Enforcement Administration said last month that its 4,200 employes will soon face mandatory urinalysis.

*The Miami Herald announced last month that all employes would be required to submit samples for drug testing before being hired; the Los Angeles Times adopted such a program in November and may also test employes who are suspected of having a drug problem, and the New York Times and Chicago Tribune have for years screened all new employes' urine for drug use. The Washington Post does not screen employes or job applicants for drug use.

When the Kansas City Times and Star revealed plans last month to send drug-sniffing dogs into the newsroom, company executives said the dogs were deemed to be less intrusive than urine samples. Nonetheless, the dogs were called off after a storm of protest.

*The Baltimore Orioles last week became the first professional baseball team to institute voluntary drug testing. Last summer, random urinalyses were begun on all minor league baseball players and front office employes under order of the baseball commissioner. Since 1983, professional basketball players have been tested if coaches suspect drug use.

The New England Patriots had planned to institute the National Football League's first voluntary testing program, but the deal was canceled last week after the names of players who allegedly have drug problems were revealed to the press.

*The Supreme Court last week cleared the way for a testing program for more than 200,000 railroad employes who have violated work rules or who work on trains that have been involved in accidents.

*Federal Express, TWA, Greyhound and Exxon, among other major companies, require urinalysis tests of all job applicants. In the District, school bus drivers and police and Metro employes are subject to drug testing. Since 1982 Pepco has been taking urine samples from job applicants and testing employes who are suspected of having a drug problem; Washington Gas Light Co. started a screening program for job applicants last February and since the mid-1970s has been testing employes suspected of being high on the job.

*Under the surprise urine testing program conducted yearly by the Coast Guard on its 38,000 men and women, monitors of the same sex accompany each individual into a bathroom and witness the procedure. "We don't want them to bring in baby's urine," said Rear Admiral Henry H. Bell, chief of Coast Guard personnel.

Accuracy problems -- including shoddy lab practices and failure to use a second test to confirm initial results -- have plagued many drug screening programs.

In recent years, the Department of Defense fired numerous outside labs and revamped its own facilities after learning that samples were mixed up and tests conducted incorrectly. In 1984, the Army notified at least 60,000 soldiers that their positive drug tests may have been wrong.

Although test manufacuturers advise that test results be confirmed through a second method, not all labs do so. Casey Triblo of Brighton, Mich., was fired from his job on a Detroit ambulance crew in 1981 after two unconfirmed tests showed evidence of marijuana use, which Triblo denied.

"I talked with a guy . . . who runs the state drug testing for racehorses, and they confirm any positives," said Triblo, who has sued the city and the laboratory. "It seems to me that if they give a damn racehorse the scientific validity of testing his urine with a confirmatory test, that's the least they could do for me."

The newest test to cause worker concern is one that detects exposure to the HTLV-III virus, which causes AIDS. In November, ENSERCH Corp. of Dallas, the parent company of a major Texas utility, became the first company in the nation to institute a regular screening program when it required cafeteria workers in one division to submit to a blood test to determine if they have been exposed to the virus. Several other Texas companies have followed suit, according to Robert Holt, a Dallas lawyer who represents two ENSERCH workers who tested positive. The employes have been continued on the payroll but ordered not to come to work.

In a case at Hendrick Medical Center in Abilene, Texas, a cafeteria worker was fired in September after he volunteered to donate blood and a screening showed the presence of the HTLV-III antibodies. "You could understand a company maybe getting bad medical advice, but a hospital where there are supposedly trained medical professionals is just incredible," said Holt.

Jim Graham, administrator of the Whitman-Walker Clinic, a District clinic serving homosexuals, said it is widely suspected that employers are screening blood taken in physicials. "We fear the test is being done and no one is being told," he said.

Dr. David Landers, director of a Los Angeles firm that sells medical screening services to corporations, said that after the military announced its testing program, six private employers asked him to begin testing their employes for HTLV-III antibodies. "All of them backed down when I asked them to develop a legal permission form to notify employes," he said.

Federal health officials have said persons who test positive for HTLV-III antibodies pose no risk to others.

But ENSERCH spokesman Howard L. Matson, who noted that the company tests cafeteria workers for exposure to other communicable diseases, such as hepatitis, said medical knowledge about how AIDS is transmitted is so "minimal" that the company does not want to take the risk of employing food handlers who have been exposed to the virus. However, he said, "We have not released anyone because they have AIDS or because they test positive for AIDS antibodies, nor do we intend to."

Although once feared, genetic and susceptibility screenings are not widely used. Dr. Bruce Karrh, medical director of DuPont, said the firm abandoned a pilot blood testing project at a New Jersey plant five years ago because the tests were no more helpful than physicals.

"These tests need to be perfected better in the research community before we could use them," Karrh said of the tests, which checked for enzyme deficiencies that might indicate future susceptibility to lung disease or anemia. Tony Mazzocchi, founder of the Committee for Responsible Genetics, a Boston-based group that monitors genetics issues, said the testing "is waiting in the wings, but it will emerge. The growth of medical surveillance firms worries us, that genetic screening could be done and you wouldn't know it."

Concerns about errors and privacy have led lawmakers in a few areas to begin to restrict tests.

Last year, San Francisco adopted the first ordinance in the country barring employers from using urinalysis to detect drug use, except where there are reasonable grounds to believe that an employe is impaired and presents a "clear and present danger" of harm.

California has a similar bill pending, and a measure may be introduced soon in the Maryland House of Delegates that would permit employers to use only tests that measure whether employes are high on the job, rather than testing past usage. Maryland already prohibits genetic testing by employers.

Under a measure passed last April, California employers cannot fire or refuse to hire employes because of HTLV-III exposure. In Wisconsin, employers cannot use the test for hiring purposes, and New York is considering a similar measure. Lawyers for the District have interpreted existing city laws as barring employers from discriminating against those with AIDS or AIDS exposure.

As is usual in American life, as more employes are subjected to various tests, more of the cases are ending up in court.

"The first wave of lawsuits is by public employes who claim that they have been discharged because of refusals to take drug tests or as a result of the tests," said Rothstein. "Looming on the horizon are a whole series of additional lawsuits brought by either applicants for public employment or in the private sector from employes who have been discharged."

Public employes generally claim that randomly subjecting them to urine tests violates their rights to be protected against unreasonable search and seizure, and to due process. Private-sector workers are employing a host of legal theories, from invasion of privacy to violation of a federal law preventing discrimination against the handicapped.

The results have been mixed. A New Jersey court in September approved a random testing program for jockeys; a New York court the next month blocked a program requiring all teachers seeking tenure to submit to a urinalysis.

In the District, the D.C. Court of Appeals in November upheld the police department's power to require urinalysis tests of officers suspected of using drugs and to discipline those who refuse to take the test. A suit on behalf of school bus drivers was dismissed on technical grounds and is now being appealed.

In San Francisco, two former employes have sued the Southern Pacific Transportation Co. as a result of its drug screening program. One, computer programmer Barbara Luck, was fired in July after she refused to provide a urine sample in a random testing.

"Her feeling was there was no problem with her work, no question of her competence," said Luck's lawyer, Lucas-Wallace. She said Luck does not use drugs: "She's a squeaky clean client."

The second employe, office manager Raymond Pettigrew, was ordered to enroll in a month-long, 24-hour-a-day drug rehabilitation program, and later to attend sessions three times weekly after a urine test showed evidence of cocaine use.

Pettigrew repeatedly denied that he used cocaine, and a second test, five days later, showed no traces of the drug. His doctor at the treatment program found "no evidence to confirm any suspicion of chemical dependency." Pettigrew was demoted after he refused to continue attending the rehabilitation sessions.

Southern Pacific spokesman Robert Taggart said Luck was fired for insubordination. He said the company is convinced that Pettigrew was using cocaine on the basis of the first test, which he said was confirmed with two other tests.

Since the testing program -- aimed primarily at employes involved in accidents -- was started in August 1984, Taggart said, the railway has experienced a 72 percent drop in the number of accidents; on-the-job injuries and sick days have also been reduced.

Other employers have had similar positive results and dismiss complaints that drug testing is unfair.

"We all live according to rules set by our employer," Taggart said. "Our employer says when we have to get to work and when we can leave and when we can eat lunch. We also say when you're working for us, you can't have drugs in your system." Urinalysis: How it Works

*Most testing programs are run by private labs that may test a variety of medical samples.

*The most widely used drug test is a Syntex Corp. test known as EMIT. The test has been shown to be 92 to 95 percent accurate and can be programmed to test for a variety of legal and illegal drugs, including marijuana, cocaine, PCP, heroin, barbituates and opiates.

At the lab, a mixture containing a chemical that reacts to the specific drug is added mechanically to the urine bottles. The level of the drug's presence is then calculated automatically by a computer that "reads" the amount of light absorbed in the reaction.

*A portable system is available, with a suitcase-sized version of the analyzer, but it can tell only if drugs are present and not specific amounts.

*The test costs between $4 and $10, depending on volume.

*If the test is positive, the manufacturer and many employers require that a second test be run, a gas chromatography analysis that can cost between $40 and $100.

*Because every person's system breaks down drugs differently, some heavy users will continue to test positive two months after their last use. Other drugs, such as amphetamines, pass through the body so quickly that they may not show up.