The company hopes to kick-start use of VeriChips by donating about 200 of the $650 scanners to trauma centers. The chips, which are the size of a grain of rice, will cost about $200 apiece. The devices are injected with a syringe under the skin of the upper arm in a quick, painless procedure.

The accompanying scanners and software ensure that the personal information unlocked by the 16-digit code is only available to those designated by the patient, Silverman said.

Opponents argue that the medical benefits are marginal at best. Patients can already wear bracelets that alert doctors to their identities and special medical needs, and few medical errors are actually caused by patients being misidentified, they say. But the potential for abuse is great, they caution.

"Over the long haul, any place where there's a surveillance camera today, five or 10 years from now will have these . . . readers. You'll walk into a 7-Eleven, and they'll take your picture and scan your number," said Richard M. Smith, an Internet security and privacy consultant in Boston. "If we start carrying these tags it makes a perfect way, either by private security companies or the government, to keep track of us."

Marc Rotenberg, executive director of the Electronic Privacy Information Center in Washington, said he was concerned that people might be forced to get the implants.

"When you put an identification tag under a person's skin, you make it impossible for a person to remove the tag, much like branding cattle," Rotenberg said. "The most likely applications would involve prisoners and parolees, and perhaps, one day, persons in the United States who are not citizens. I think there needs to be some legislation put in place to prevent abuse."

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When the U.S. Food and Drug Administration approved implanting microchips in humans, the manufacturer said it would save lives, letting doctors scan the tiny transponders to access patients' medical records almost instantly. The FDA found "reasonable assurance" the device was safe, and a sub-agency even called it one of 2005's top "innovative technologies."

But neither the company nor the regulators publicly mentioned this: A series of veterinary and toxicology studies, dating to the mid-1990s, stated that chip implants had "induced" malignant tumors in some lab mice and rats.

"The transponders were the cause of the tumors," said Keith Johnson, a retired toxicologic pathologist, explaining in a phone interview the findings of a 1996 study he led at the Dow Chemical Co. in Midland, Mich.

Leading cancer specialists reviewed the research for The Associated Press and, while cautioning that animal test results do not necessarily apply to humans, said the findings troubled them. Some said they would not allow family members to receive implants, and all urged further research before the glass-encased transponders are widely implanted in people.

To date, about 2,000 of the so-called radio frequency identification, or RFID, devices have been implanted in humans worldwide, according to VeriChip Corp. The company, which sees a target market of 45 million Americans for its medical monitoring chips, insists the devices are safe, as does its parent company, Applied Digital Solutions, of Delray Beach, Fla.