Sen. Edward M. Kennedy, one of the main designers of the federal anti-cancer effort, urged government health officials yesterday to test Laetrile in large numbers of cancer patients to learn its worth one way or the other.

He simultaneously urged Laetrile's backers to agree to stop promoting the drug and "raising false hopes" and "misleading the American people" if it proves valueless.

At a four-hour session of his Senate Health subcommittee - with a confrontation between Laetrile's supporters and detractors - the Massachusetts Democrat won agreement on that point from some of the backers.

But they quarreled with the federal officials about what kind of test would be "fair" and valid. Members of both sides later agreed that they may never agree on any kind of test that the other side would not call meaningless.

More than 50,000 Americans with cancer are now being treated with Laetrile imported from Mexico in one way or another. Dr. Donald S. Frickson, director of the government's National Institutes of Health, testified.

But only animal tests have been done so far to try to assess scientifically Laetrile's anti-cancer value.

The subcommittee first heard uniformly unfavorable testimony on the controversial chemical from government and other scientists, testimony based on the animal tests and review of the cases of a number of uncured Laetrile patients.

The Food and Drug Administration's commissioner, Dr. Donald Kennedy, said lack of demonstration of "any useful effect" as well as "evidence of fraud" and huge profiteering by Laetrile backers fully justify the FDA's prohibition of the drug.

Kennedy, Fredrickson and Dr. Joseph Ross of the University of California at Los Angeles said Laetrile can poison a patient when taken by mouth, a from of treatment often advocated.

Dr. Lewis Thomas, president of Memorial Sloan Kettering Cancer Center in New York, saw "not a particular scientific evidence" that Laetrile works. But he called it essential that the public get "solid information" and urged both a study of the fate of patients give Laetrile so far and a test on cancer patients at least three major medical centers.

Frederickson promised a decision shortly on whether NIH's National Cancer Institute will sponsor such a trial. Thomas said it would be unethical to withhold other treatments from cancer patients but Laetrile could beadded to see whether it prolongs life or relatives pain.

Laetrile proponents, however said ordinary treatments often harm patients by "cutting, burning or poisoning" - their way of referring to surgery, radiation and chemical therapy - and could counteract Laetrile's effects.

They themselves made far more modest claims for Laetrile's effects than Laetrile's friends were making a few years ago.

Robert Bradford, president of the Committee for Freedom of Choice in Cancer Therapy, biochemist Ernest Krebs Jr., Dr. John Richardson and Dr. Bruce Halstead agreed that Laetrile does not "cure" cancer although, they said, it may "control" it.

They said Laetrile's main value could be to prevent cance, describing it as "vitamin B-17" The same substance, said Krebs, is found in lime beans, millet, buckwheat, bamboo shoots and other foods that everyone should eat in quantity, he maintained, to prevent the "nutritional deficience" that causes cancers.

This contention, too, was disputed by the scientists. They said there is no firm evidence that cancer is a nutritional disease and none at all that Laetrile is a vitamin.

Of the four Laetrile backers who testified, three have been convicted on federal or state charges of smuggling the drug into the country or promoting it.

Bradford and Richardson were convicted in federal court. Bradford was fined $40,000 and Richardon $30,000, and Richardon also lost his California medical license. Krebs was convicted of violating California health laws and is free on bond pending on appeal.

All claimed yesterday that they were persecuted. They denied federal and California prosecutors' testimony that they and others have made sums running into the millions by Laetrille distribution and promotion.