A white powder that looks like sugar, acts like sugar and tastes like sugar -- but adds no calories -- may sound like a sweetener that is out of this world.

Local entrepreneur Gilbert V. Levin did test the product on Mars before getting a patent on Earth but thinks the response here may be more favorable and hopes it will someday fatten the profits of his Rockville company, Biospherics Inc.

Such a substance would be a treat for dieters and diabetics and even would help prevent tooth decay, the company says, setting its sights on the estimated $900 million-a-year market for low-calorie sweeteners.

"We believe that the public's demand for substitute sweeteners is virtually limitless," said Levin, president and founder of Biospherics, who first used the substance in an experiment he designed to test for life on Mars.

If it sounds too sweet to be true, Levin acknowledges that his small company needs the help of a "big brother" corporation with the money, technology and marketing ability to help finance the transition between an inventor's dream and worldwide sales. Biospherics, with 1984 net income of $110,325 on revenue of $6 million, "is negotiating with several large corporations," he said.

Levin's product, Lev-O-Cal, resembles sugar so closely because it is sugar -- but it is a form of sugar that our bodies cannot digest. Known as "left-handed" sugar, or L-sugar, it is chemically identical to common sugar but has a slightly different molecular configuration -- a form that is a mirror image of the more familiar substance.

Our taste buds recognize it as sweet, but our digestive enzymes don't recognize it at all. These enzymes have evolved so that they only react with "right-handed" sugars, which are found in many commonly occurring forms. When they find a left-handed sugar, "it is like trying to fit a left hand into a right-handed glove," Levin said. Life Chose Right-Handed Sugars

No living creature is known to metabolize L-sugars, so even the bacteria that promote tooth decay would be unable to process the stuff, Levin said, adding, "For some reason, life selected right-handed sugars."

Life probably made the right choice. Right-handed sugars occur widely in many forms, including sucrose, or table sugar, which comes from sugar cane, sugar beets, molasses and maple syrup. Other forms, such as fructose and glucose, are found in fruits, vegetables and honey.

L-sugars are much rarer, appearing in seaweed, plaintains, algae, snail eggs, sugar-beet pulp and some other plants.

But diet-conscious life forms today are choosing to consume less sugar. Sugar use in this country declined to 8 million tons in 1984 from 10.1 million tons in 1979, according to the U.S. Department of Agriculture. Over the same period, the use of low-calorie sweeteners such as saccharin and aspartame climbed.

Levin also believes his product has advantages over other low-calorie sweeteners. Lev-O-Cal has the texture and bulk of sugar essential for ice cream and behaves like sugar when heated, retaining its taste and browning in baked goods, he said. Idea Came From Left Field

Levin said the idea for Lev-O-Cal "really came from out of left field," while he was studying organic chemistry in the early 1960s for his Ph.D. in environmental engineering at Johns Hopkins University. Cyclamates then dominated the artificial sweetener market, but evidence linking them with cancer emerged and later led to a ban on their use in 1970. "I thought of it then: Why not use left-handed sugar?"

He put the idea aside for a few years, concentrating instead on a career in environmental health that led to the founding of Biospherics in 1967, as an environmental engineering company focusing on problems of air and water pollution, space biology and oceanography. Levin's first chance to use left-handed sugars arose out of the company's first contract, an assignment in 1967 to write a proposal for a life-detection lab for the Voyager mission to Mars.

NASA later canceled that mission, launched another named Viking, and chose Biospherics to contribute one of three biological experiments designed to look for signs of life.

The Biospherics experiment was the only one that showed positive results, the meaning of which continue to be debated by some members of the scientific community.

"I never claimed we found life on Mars," Levin said. "I claimed we detected evidence . . . that there is a good probability of life on Mars."

Levin, chief scientist in charge of that experiment, designed a radioactive nutrient mix that was combined with a tiny amount of Martian soil. The experiment then measured an emission of radioactive gas, which may have been produced by living microorganisms or may have been the product of a chemical reaction, according to different interpretations of the results.

While developing the nutrient mix, Levin decided to use left-handed as well as right-handed sugars. "I thought maybe the Mars bugs might eat left-handed sugars," he said.

He tasted a non-radioactive sample and was surprised by the results. "It was sweet. . . . That was a remarkable finding," he said.

By 1981, Levin had patents on the use of L-sugars as sweeteners. The next problem was production cost -- about $30,000 per pound at the time. When a major baked goods company expressed interest and asked for a free two-pound sample, Levin couldn't provide it.

Now Biospherics has the rights to a process that would make one type of L-sugar, l-gulose, for less than $1 per pound, Levin said. The company also has applied for a patent on a process that would make L-fructose. But it lacks the money to produce either form in large quantities.

The company currently plans to make Lev-O-Cal through chemical synthesis, which mixes organic chemicals under certain conditions to alter their molecular structure. Beet pulp or corn would be possible raw materials.

"Maybe in the future," Biospherics might produce its product through gene splicing or other new genetic engineering techniques, Levin said. The company is talking to biotechnology firms, but thinks it would take too long and cost too much to pursue that route now, Levin said. Approval Process Eats Up Big Bucks

Biospherics also lacks the big bucks needed to conduct the testing to win Food and Drug Administration approval of a new food substance. The company has fed Lev-O-Cal to rats on a limited basis, finding no adverse effects and obtaining "evidence indicating that little, if any, L-sugar is metabolized by the animals."

Levin predicts it will take about $13 million and three to five years to bring the product to market. Until then, the company will continue to rely on income from its laboratory, engineering and communications divisions.

Biospherics performs contract laboratory work, analyzing and identifying pollutants, toxic wastes and industrial hygiene hazards. The engineering and instrumentation division produces products and processes to address pollution problems. Most of the company's revenue, 54 percent in 1984, was generated by the communications division, which provides information services.

The stock market recently began showing an increased taste for the company. In November, the stock was selling at $2 a share, far below its five-year high of $7. Then, over the course of the next three weeks, Biospherics' stock price rose 112.5 percent to $4.25, a gain that may be due to a favorable article about the company in Fortune magazine during that period. The stock closed Friday at a bid price of $4.50.