"Who controls the data" collected from RFID tags? Albrecht asked. She worries that companies putting tags into consumer products might forge alliances with the makers of carpeting, for example, to embed sensing devices that could develop intelligence about how consumers use the items.

Industry dismisses those kinds of scenarios as paranoia, but Albrecht and other activists have forced companies to pay attention to them. A store in Rheinberg, Germany, took RFID tags out of its loyalty cards after protests. Many large firms working with RFID now have extensive disclosure statements on their Web sites.

"Anonymity is an important issue that must be handled very thoughtfully," said Elliot Maxwell, who heads an international committee that advises EPCGlobal on privacy and other policy issues.

But he also recognizes the RFID paradox: "In order to have the most value to both individuals and society, the infrastructure [to read tags] needs to be widespread," he said, citing medical monitoring and the ability to track toxic products, or stolen guns, as examples. "And yet it is just that widespread infrastructure that raises the most questions."

Tags cannot be read at more than about 20 feet, but many say that reading capability will rapidly advance. And given RFID's potential to track stolen goods, privacy activists wonder how long it will be before tags are embedded in money.

But few applications raise more eyebrows than RFID tags implanted in people, a business pursued by Applied Digital Solutions Inc. and its subsidiary, VeriChip Corp., of Palm Beach, Fla.

The company has for years provided rice-grain sized tags for implants into pets and cattle. But it made waves two years ago when a Boca Raton man, his wife and 14-year-old son agreed to let the tags be implanted in them.

The company and the family hoped the tag would speed patients through frequent hospital visits or in the case of an emergency by quickly alerting doctors to a person's identity and medical history. But the FDA quickly stepped in and deemed medical uses of the technology subject to government approval, which is still pending.

Scott R. Silverman, Applied Digital's chief executive, said he hopes the FDA will provide clearance by the end of the year.