WASHINGTON - The Humane Society of the United States sued the Agriculture Department on Wednesday for creating a “loophole” that it said is permitting potentially sick cows into the food supply.

The lawsuit, filed in federal court in Washington, accused the department of violating procedural requirements when it created the provision, giving the meat industry a financial incentive to send unhealthy cattle to slaughter. As evidence, the Humane Society cited its widely publicized undercover videotape of workers at the Westland/Hallmark Meat Company in Chino, Calif., abusing cows that appeared unable to walk.

Jonathan R. Lovvorn, a lawyer for the Humane Society, said that when the agency weakened the ban last year, it “did so without really telling people that that’s what they were going to do and without explaining how this complies with their obligation to protect consumers and ensure humane treatment.”

The lawsuit is likely to fuel tensions between the Humane Society and the Agriculture Department that have been growing since the video was publicly released in late January. The release led to the biggest beef recall in history, 143 million pounds, more than a third of which had been shipped to federal nutrition programs like school lunches.

The agriculture secretary, Ed Schafer, has criticized the Humane Society for not giving his agency the tape soon after it was shot, in October and November. The Humane Society said it took the tape to local prosecutors in mid-December because of the animal abuse, asserting that the Agriculture Department had ignored past abuse.

Through a spokeswoman, Mr. Schafer, who was named personally in the suit, declined to comment. A second defendant, Alfred V. Almanza, administrator of the agency’s Food Safety and Inspection Service, also declined to comment. They have previously expressed confidence in the food supply and argued that the Westland/Hallmark case was an isolated problem.

Much of the recalled meat has already been eaten. The Agriculture Department said that no one had become ill and that the health risk was minimal.

An investigator for the Humane Society spent six weeks working in the outdoor pens at Westland/Hallmark, which used spent dairy cows to make ground beef. The Chino area has many large dairies.

During that time, he shot video of workers using forklifts and electric prods to force cows to their feet in an apparent effort to get them to walk. In one instance, a worker sprayed a hose down a cow’s nose to get it to stand up.

The Agriculture Department tentatively banned cows that cannot walk, called downers, from the human food supply in 2004 after the first discovery in the United States of a cow with bovine spongiform encephalopathy, or mad cow disease.

Downer cows have an elevated risk of certain diseases, including mad cow. Under rare circumstances, that illness can be transmitted to people, causing a fatal brain ailment.

In July 2007, after lobbying efforts by the meat industry, the Agriculture Department made the downer cow ban official, but with a change.

When cows arrive at a slaughterhouse, a Department of Agriculture veterinarian inspects them to make sure they are fit for slaughter. Under the amendment, if a cow goes down after that initial inspection, the meat company is supposed to summon the veterinarian to determine whether the animal is healthy enough for slaughter.

Mark D. Dopp, senior vice president for regulatory affairs and general counsel for the American Meat Institute, said the initial ban was too broad because it included animals that might break a leg or sever a ligament after the inspection.

The American Meat Institute was among many organizations, including meat companies, farmers and state departments of agriculture, that urged the federal department to amend the ban.

“It’s not like there would be anything wrong with the carcass, with the meat, if the animal breaks a leg,” Mr. Dopp said. “It’s a proviso founded in common sense.”

Getting rid of the provision would severely restrict the market for cows that are in poor condition, Mr. Lovvorn argued. Now, he asserted, companies like Westland/Hallmark buy feeble cows on the assumption they can make it through a lax inspection.

The Humane Society said the Agriculture Department had done a poor job of enforcing even the weakened version of its downer ban, as evidenced by the problems at Westland/Hallmark. In addition, a 2006 audit by the agency’s inspector general found that meat inspectors sometimes allowed downer cows to be slaughtered, despite the rules.

During a nine-month period in which 12 slaughterhouses were examined, 29 cows unable to walk were slaughtered at two of the plants. Of those animals, 20 exhibited no evidence of an injury that would explain why they could not walk, raising the possibility of a more serious illness.