Use of the controversial synthetic sex hormone, DES, to fatten beef cattle by the thousands was forbidden yesterday by the Food and Drug Administration, because the residues left in meat may cause cancer.

Eliminating the harmone - a powerful growth promoter used since the 1950s in the animals that provide Americans' hamburgers and steaks - could cut beef supplies by 3.7 percent, and raise meat prices by 9.1 percent in a year, Congress' Office of Technology Assessment (OTA) said early this week.

But other, apparently safer drugs may be substituted, FDA and OTA agreed. By the end of a decade, OTA said, the additional cost should amount to no more than 5 percent.

The livestock and DES industries have blocked a DES ban for the past seven years by filing suits in courts protesting in administrative proceedings.

Donald Kennedy, serving his last day as FDA commissioner, said yesterday food safety laws don't permit FDA to consider economic effects. However, he maintained, "those opposing the ban have not demonstrated that it will have any significant" economic impact.

DES - diethylstibestrol - is the same synthetic female sex hormone widely used a generation ago for pregnancy disorders. But some daughters of "DES mothers" developed vaginal or cervical cancer and some sons, genital abnormalities.

The drug is still prescribed for human use in some cases, and these will not be affected by FDA's ruling. It is estimated that 1.3 million DES prescriptions per year are writtens to treat prostate cancer in men, to treat some women after childbirth or menopause, or to dry up breast milk secretion.

In cattle and sheep, DES is used to spur growth either by implanting pellets of it behind the animals' ears, or by adding it to their feed. In grain industry terms, this makes the feed "more efficient."

But Kennedy pointed out: "DES has been shown to cause cancer in both animals and people."

Low-level residues remain in meat. No one can show there is any level, however small, that is safe. And there is no currently approved way to detect the smallest but still possibly cancer-causing residues.

FDA first issued DES bans in 1972 and 1973, but they were overturned by the U.S Court of Appeals because the agency had not first held a public hearing.

In 1976 FDA started new proceedings. The four main manufacturers of agricultural DES - American Home Products Corp., Dawes Laboratories, the Hess and Clark Division of Rhodia Inc., and Vineland Laboratories - asked for a hearing, which began in November of that year. Last September an FDA judge upheld a ban.

Kennedy yesterday said manufacture and shipment of agricultural DES must stop by July 13, and use in animals by July 20.

Massive use of another class of drugs, antibiotics, continues meanwhile to suppress disease and promote growth in livestock and poultry. OTA believes this use helps make disease germs that infect human beings more resistant by killings off susceptible strains and letting stronger ones flourish.

FDA first proposed in 1977 to ban routine use of penicillin in animal feeds, to eliminate most uses of tetracycline - the most widely used feed drugs of all, and to take action on other antibiotics. These FDA proposals are pending.

OTA has found that appropriate substitutes for these antibiotics are generally available. The initial economic impact of antibiotic bans might be significant, the OTA believes, but the long-range impact would probably be much less. CAPTION: Picture, DONALD KENNEDY . . . "DES causes cancer in people"