The action by Walmart and Supervalu, which owns supermarket chains like Acme and Jewel, was highly unusual because there had been no determination by authorities that the formula was to blame for the child’s death, and neither the manufacturer nor federal officials had sought a recall.

Shares of Mead Johnson Nutrition, which manufactures Enfamil, fell more than 20 percent intraday on Thursday as news about the situation spread. The stock partly recovered by the end of the day, closing at $68.76, down 10 percent.

The Food and Drug Administration said it was testing samples of the formula, Enfamil Premium Newborn powder, that it had obtained from the family of the dead child, Avery Cornett. Officials asserted that there was no immediate indication that the formula had been contaminated.

A Walmart spokeswoman said the company learned on Sunday night of the baby’s death and that his parents had fed him formula bought at the chain’s store in Lebanon, Mo. On Monday, the company decided to pull all containers of the formula that had come from the same manufacturing lot from the shelves of its stores.

“As a precautionary measure, we made a company decision to remove the specific lot of the product from our store shelves nationwide,” said Dianna Gee, the Walmart spokeswoman. “We’re not saying the product’s unsafe.”

Walmart identified the product consumed by the baby who died as Enfamil Premium Newborn powdered infant formula in a 12.5-ounce container, with the lot number ZP1K7G printed on the bottom of the canister.

The baby boy, who was 10 days old when he died on Sunday, was infected with Cronobacter sakazakii, a bacterium that is commonly found in the environment and has occasionally been known to cause severe illness in infants.

In a written statement, Mead Johnson said Thursday, “We are confident that all our products are safe and nutritious when prepared, stored and used as instructed on the label.”

The company said that it had tested all of its formula for Cronobacter. It said that its records showed that the batch bought by the Missouri infant’s family had been tested before it was distributed and that no Cronobacter had been found.

The Centers for Disease Control and Prevention said that it received reports of about four to six Cronobacter infections in infants a year. The fatality rate is 40 percent.

Scientific studies and disease investigations have found Cronobacter in various types of formula. In some cases, when the bacterium was found in open containers, it was not clear whether the formula had become contaminated after opening. But in other instances, the bacterium has been found in sealed packages delivered from the factory.

In one case in 2001, an infant died from Cronobacter in Tennessee, and investigators found a genetically indistinguishable strain of the bacterium in sealed containers of the formula, according to reports by the Centers for Disease Control and Prevention and the F.D.A. The formula, a Mead Johnson product called Portagen, was recalled.

Health officials said it was important for parents to take precautions when preparing formula. That includes washing hands thoroughly, sterilizing bottles and other feeding equipment and preparing only enough formula for a single feeding.

Officials said the bacterium could grow over time if it was in the formula, and holding onto mixed formula for more than one feeding could increase the chance of infection.

Ms. Gee said Walmart had removed the newborn formula from more than 3,000 stores nationwide. She said the company was holding onto the product and that if it was cleared in the investigation, the company might put it back on sale. Walmart continues to sell other Enfamil products.

Walmart said customers who bought Enfamil from the same lot could bring it back and receive a refund or another product in its place.

Supervalu said Thursday that it had also received formula from the same lot and was removing it from store shelves.

Michael A. Siemienas, a Supervalu spokesman, said the formula might have been shipped to hundreds of its stores around the country, including Jewel, Shaws, Shop and Save, Acme, Farm Fresh, Shoppers and some Albertsons supermarkets.

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