Genex Corp. of Rockville announced yesterday that it has entered into a contract with G. D. Searle & Co. to provide a key ingredient for the low-calorie sweetener Aspartame for use in the diet soft drink market.

Genex said that, by using an innovative "bio-reactor" technology, the company will be able to produce one of two major ingredients in the sweetener more rapidly and at lower cost than it has been produced in the past.

Company officials would not say how much the contract with Searle Food Resources Inc. is worth, but the agreement appeared to be the beginning of a long-term relationship. Under the contract, Genex will supply quantities of L-Phenylalanine, to be used in Searle's low-calorie sweetener, Nutrasweet, which is one of several brand names under which Aspartame is sold.

The agreement also grants Searle a license and rights of access to some areas of Genex technology that involve "sweeteners," not limited to Aspartame ingredients.

In addition, Searle received warrants to purchase 9.9 percent of Genex shares at $26 a share. Half of the stock must be acquired on or before March 31, 1985; the other half must be acquired on or before Sept. 30, 1985. Genex shares have been trading between $18 and $21 recently.

The agreement, which would amount to a $30 million cash payment if the warrants are exercised, suggests that "both Searle and Genex expect Genex stock to be selling at substantially higher than $26" when the warrants run out, said Nelson Schneider, a drug industry analyst for E. F. Hutton.

Genex closed at 21 5/8 yesterday, up 1/8.

Searle's purchases of L-Phenylalanine are certain to have an impact on Genex's earnings during the intervening 18 months, he pointed out. "We are very enthusiastic about Genex," Schneider said.

L-Phenylalanine and L-aspartic acid are both amino acids that are mixed together in roughly even portions to produce Aspartame.

Searle has bought L-Phenylalanine in the past from a Japanese company, which produces it through a relatively expensive fermentation process. Genex spokeswoman Shelley Roth said the company's process uses columns of immobilized cells to produce the same reactions generally produced using larger, slower equipment.