One of the three, the Taixing Glycerine Factory, sold an ingredient laced with a potent poison, diethylene glycol, that was later mixed into cold medicine, killing at least 100 people in Panama over the last year.

Diethylene glycol, a sweet and inexpensive substitute for glycerin that is also a prime ingredient in some antifreeze, is the same poison later found in Chinese toothpaste, prompting countries around the world to recall it.

Chinese regulators also announced that they had revoked the business licenses of two companies that exported tainted wheat protein used in pet food. An undetermined number of animals in the United States died as a result.

The two companies, the Xuzhou Anying Biotechnology Development Company and the Binzhou Futian Biotechnology Company, evaded inspections by labeling their product as chemicals, which are not subject to compulsory inspection in China, according to Xinhua, the state-controlled news agency, citing comments by government officials.

And, China acknowledged that several Chinese companies had exported seafood tainted with banned antibiotics to the United States.

Regulators said the seafood suppliers were not properly registered with the government’s quality inspectors. China’s announcements came amid a flurry of government activity this month aimed at combating growing criticism of the Made-in-China label, including reports about defective tires, exploding cellphone batteries and toys coated with lead. Friday’s actions, though, took direct aim at the companies implicated in two scandals with particular resonance.

“The Chinese government pays great attention to addressing flaws in product quality, especially the quality of food products,” Li Changjiang, a top official at the General Administration of Quality Supervision, Inspection and Quarantine, said at a Beijing news conference on Friday, according to Xinhua.

Beijing has adopted a double-barreled response. On the one hand, the government promises sweeping changes in food and drug regulation. It has begun nationwide quality and safety inspections and recently banned the use of diethylene glycol as a thickening agent in toothpaste. Last week, it executed the former head of the State Food and Drug Administration, convicted of corruption charges.

At the same time, Beijing has disputed claims that Chinese exports are particularly dangerous, insisting that 99 percent of its exports meet quality standards and accusing the foreign media of exaggerating the extent of safety-related issues.

The government expressed new interest in the Taixing Glycerine Factory after The New York Times in May traced the Panama poison to the factory, near the Yangtze delta in eastern China.

The company had falsely labeled the poisonous compound as 99.5 percent glycerin, a safe ingredient used in food, medicine and other products, records show. The Times found that the factory was not certified to make glycerin.

Afterward, Chinese authorities reopened a dormant investigation of the company, culminating in its decision Friday to close the Taixing Glycerine Factory.

“The Food and Drug Administration welcomes any action by Chinese authorities that will help ensure the safety of the products that are made in China,” said Julie Zawisza, a spokeswoman for the Food and Drug Administration.

By Friday night, the company’s sign at the factory’s entrance had been taken down. “The factory has gone out of business,” said Ding Yuxi, who worked at the company for 10 years and was guarding the compound.

At the news conference in Beijing, Mr. Li, the Chinese regulator, said nothing about any criminal prosecutions involving the factory, nor did he mention the state-owned company, CNSC Fortune Way, that exported the poison with false papers.

Although regulators said they would no longer allow products with misleading labels, Chinese officials still blame a Panamanian importer for changing the product’s paperwork to make it appear more suitable for human consumption. The F.D.A. said no one would have died if Chinese companies had properly identified the product’s contents.

Dr. Jorge Motta, a prominent Panamanian physician who helped identify the poison that was killing so many patients, said the plant’s shutdown was overdue.

“Every day that this factory remained open made me fear that this terrible problem that occurred to us could occur to someone else,” Dr. Motta said. “Knowing that people responsible for all the suffering that we had in Panama continued to function without any questioning was not only painful but also an insult to my country.”

With President Bush and Congress now pressing for stricter controls on United States imports, China is not only announcing crackdowns on unsafe products. It is also taking its case to the negotiating table. Next week, European Union officials are expected to meet for high-level talks about the quality and safety of China’s exports. Less than a week later, American officials are expected to arrive in Beijing for meetings aimed at improving quality and safety inspections and to resolve what has begun to look like a trade dispute between the countries.

Late last month, the F.D.A.. blocked the importation of some Chinese seafood. China responded last week by banning some pork and chicken products from American producers, saying they, too, were tainted with banned chemicals.