One day more than a year ago, Carolyn Kramer was putting away her son's clothes when she found 20 black capsules in a coat pocket. Alarmed, she and her husband, a doctor, tried to identify the pills in his Physicians' Desk Reference.

The size, shape, even the numbers on the pills were remarkably similar to a well-known amphetamine nicknamed Black Beauties, available only through prescription. Nothing in the book, however, exactly matched the capsules the Kramers found in their 16-year-old son's coat.

"He didn't know what he was using," said Kramer, who is convinced her son was fooled into thinking he was buying speed, the slang term for amphetamines. "He thought he was taking the real thing."

This week, Carolyn Kramer and 10 other Fairfax County parents visited the Virginia General Assembly to lobby for bills that would ban the sale of so-called "lookalike" drugs--legal drugs manufactured and marketed to look like the illegal "uppers" and "downers" popular among teen-age drug users. Del. James H. Dillard II (R-Fairfax), sponsor of one of the bills, said today he expects the legislature to approve some measure to control the phony pills.

Lookalikes are so pervasive that, according to Dr. Mel Riddile, coordinator of the substance abuse program in Fairfax County schools, only one out of every 10 pills sold in the drug underground is what buyers think it is. Bottles of 25,000 are available for $30, although the pills often are sold for as much as $2 apiece.

The imitations--called Pink Footballs, Robin's Eggs, Pink Hearts--contain caffeine and commonly used nasal decongestants, but the packaging does not resemble any over-the-counter cold remedies. Riddile said he has seen teen-agers buy pills they think are Quaaludes, a sedative, because the coding on the lookalikes--Lemon 714 or Lennon 714--is deceptively like Lemmon 714, the inscription on a brand-name prescription drug.

At a hearing before the House Courts of Justice Committee, parents repeatedly pleaded for controls on lookalike drugs, which they said are enticing young people into the drug culture at enormous profit to fly-by-night distributors and at some risk to the consumer.

The U. S. Food and Drug Administration supports state laws banning the sale of lookalikes. Although the ingredients are safe if taken in limited quantities, teen-agers often down five, six or more at a time, looking for a high they'd expect from amphetamines or other drugs.

The FDA said the effects can be lethal, particularly among people with a tendency to high blood pressure. At least 15 deaths nationwide have been traced to lookalike drug use, according to FDA spokesman Christopher Smith.

School officials have said the suggestive effects of the pills also are a problem: "Students believe they're taking speed and their behavior is close to what it would be if they had. They become animated, talkative, restless," said Riddile.

Parents are concerned that their children, misled by the mild effects of the lookalikes, may be tempted to take an overdose of amphetamines or barbiturates, particularly if they can't tell the real from the fake.

"It's plain old ordinary rip-off," Smith said. "We all know why they are being manufactured that way, made to look that way, are numbered that way. I can't imagine why some of the kids haven't caught on."