When Discovery lifts off Friday on the 16th space shuttle mission, it will carry an experimental device to grow crystals of 34 proteins that can't be grown on Earth.

The endeavor is no idle laboratory curiosity. If science can make crystals large enough to be X-rayed, mapped and analyzed, the nation's drug manufacturers will have a major new tool for developing medicines to treat illnesses caused by viruses and bacteria.

"They're not kidding when they talk about curing the common cold," Richard E. Halpern, director of microgravity sciences for the National Aeronautics and Space Administration, said in an interview.

"Microgravity science" might be defined as making things in the near-weightlessness of space that are not made well on Earth, and the crystal-growth effort is one of an increasing number of exotic experiments in the field.

Metals, fluids, crystals and living cells have a common drawback in the presence of gravity. They are difficult to isolate and purify while gravity pulls them with the same force as it does their contaminants. They tend to settle out with their contaminants or to carry contaminants with them if they bubble to the top while being treated with heat.

As a result, enzymes such as urokinase -- used to dissolve the blood clots that kill 50,000 people a year in the United States alone -- are so difficult to obtain in pure enough form on Earth that they cost $1,500 a dose.

Equally difficult to produce are exotic semiconductors, such as gallium arsenide, which are known to use less power and switch electrons 100 times faster than the silicon used in most microchips.

Gravity also prevents the laboratory growth of all but 100 of the estimated 500,000 proteins involved in the diseases that ravage mankind.

The Discovery experiment -- designed by the University of Alabama, Vanderbilt University and the University of Pennsylvania -- is designed to break gravity's hold preventing the development of new medicines.

"If the proteins at the heart of most viruses and bacteria can be enlarged to reveal their structure, then these dreams of treating diseases from arthritis to influenza are real," Halpern said.

Two experiments to grow crystals in weightlessness were performed 18 months ago on the first flight of the $1 billion Spacelab built by the European Space Agency. One protein grew to be 27 times bigger than any such crystal on Earth; the second, to 1,000 times its earthbound size.

The experiments triggered such a flood of interest by such drug companies as Merck, Schering-Plough, Upjohn, Smith Kline & French and Burroughs-Wellcome that Halpern calls protein crystal growth the "hottest thing we've got going right now."

Crystal growth isn't the only thing going on in microgravity, however.

Samples of urokinase flown on a previous space shuttle flight were found to separate at better than six times the rate they do on Earth. Halpern hopes to double that rate on a future flight.

Gallium arsenide is also a future shuttle candidate. Grumman and the Florida firm Microgravity Research Associates have approached NASA about financing such experiments.

Metals are prime candidates for space manufacturing. Among the companies that have talked to NASA about reserving shuttle space for metals experiments are Bethelehem Steel Corp., Alcoa, Specialty Metals Corp., John Deere & Co. and Ford Motor Co.

General Motors is interested in learning how to cast lightweight alloys in space for car bodies.

A prime example of a metal that may find a home in space is the strategic material iridium. Iridium, which doesn't melt until it reaches a temperature above 3,000 degrees Fahrenheit, lends strength to metals ranging from aluminum to steel. NASA has signed a "technical exchange agreement" with Argonne National Laboratory to see if iridium can be purified in space.

Two of NASA's most ambitious experiments in microgravity are under way, both of them secret except to the companies financing them. "Even I don't know what they're trying to produce," Halpern said, "and that's the way I want it to stay."

One experiment, financed by the McDonnell Douglas Corp. and Johnson & Johnson, will make its fifth flight Friday on Discovery. It involves the separation of biochemical fragments to produce the pure form of an unidentified hormone the two companies say they will use in the experimental treatment of incurable cancers.

At least one of the previous four efforts was a failure, but the two companies have been so cheered by their successes that they have reserved space in the shuttle's cargo bay in November for a miniature biochemical "factory" 10 times the size of the unit they've been flying in the shuttle's cabin.

The other current experiment is financed by the 3M Co. and involves an effort to grow super-pure crystals that can be spun like glass into optical fibers. The fibers would then be wound into light-transmitting cables that might carry 1,000 times the number of telephone calls carried by copper cables today.

3M has not identified the crystals it has grown on one flight and plans to fly again later this year. "It's their business, not ours," Halpern said -- though he hopes it will be NASA's business for years to come.

The second flight of the European Spacelab, set for April 29, is a seven-day mission dedicated to microgravity experiments. So vital are Spacelab's experiments that its seven-man crew has been told to limit in-flight exercise so their movements won't create the small amount of gravity that could disturb the experiments. The astronauts who run the experiments have been ordered to move their hands away from the experiment racks if they feel a sneeze or cough coming on.

Marine Col. Robert F. Overmyer, who will command Spacelab, said: "I've been told to keep my hands off the controls and let the autopilot do the flying. If I try to fly the shuttle manually, it will trigger enough course changes to introduce artificial gravity to disrupt some of our experiments."

Planned for an August shuttle flight is an experiment involving the separation of blood cells. Gravity has such a strong effect on blood cells that blood platelets can be stored only for five days and whole blood for 30 days.

"Blood tends to settle out badly, forming a sediment that damages platelets and individual red or white cells and destroy their usefulness in a short time," Halpern said. "Maybe our August flight can get around that."